Sen. Mark Hatfield (R-Ore.) heads a group that includes Republicans Strom Thurmond (S.C.), Roger Jepsen (Iowa), John Chafee (R.I.), Pete Domenici (N.M.), Richard Lugar (Ind.), Charles Percy (Ill.), Larry Pressler (S.D.) and Jeremiah Denton (Ala.) and Democrats Sam Nunn (Ga.), Jennings Randolph(W. Va.) and Lawton Chiles (Fla.) among many others.

"We do have a pretty broad base," says Hatfield. Indeed: An average of 25 senators meets regularly on Wednesday mornings to share their private faith in this most public of buildings, under the massive cut-glass chandelier of the Vandenberg dining room. Each week one senator is chosen "to address in total openness a subject of importance that has a spiritual dimension," Hatfield says. "One of the best things it has going for it is that it's totally off the record -- no one is ever there other than a senator.

"I don't recall ever having discussed a pending bill or pending amendment," says Hatfield. "Nobody's trying to push anything -- that would kill it. A spiritual association of this kind is a personal, relational experience rather than a political exercise."

"I wouldn't say that you never hear political issues discussed," says Sen. William Armstrong (R-Colo.), "but it is organized for other purposes. Any attempt by any of us to turn it to political advantage would be unthinkable." Does that mean that when the Senate voted last month to prevent the Justice Department from stopping "voluntary prayer" in public schools, the subject wasn't mentioned? "Never," says Armstrong. "I can't recall a single instance where the prayer breakfast has ever had any lobbying activity." Coming Out of the Closet

Since the election of Jimmy Carter and the eruption of public interest in evangelical Christianity (a 1976 Gallup poll estimated that 50 million of the voting-age population had had a "born-again" experience), shifting social pressures have pushed political groups such as Moral Majority into prominence. But at the same time, other and less visible Christian forces have arisen in and around the Senate.

With the encouragement of individual pastors or under the auspices of organizations such as Christian Embassy (a wing of Campus Crusade for Christ, with five full-time employes ministering to Congress) and the Fellowship Foundation, regular prayer groups are commonplace. The senators' breakfast, which has been meeting for decades, is larger than it has been in years, says Hatfield. An estimated 800 of the 8,000 Senate staff are active in dozens of weekly evangelical gatherings; Dee Jepsen leads a weekly Bible-study meeting for Senate wives; a large group of judges meets weekly in the same room the senators use; and weekly groups meet almost daily at the Pentagon and State Department.

So next year, when Congress takes up issues such as abortion, busing, school prayer and tuition tax credits for parochial schools, it will do so in a context of growing evangelical activity. And in a climate of changing attitudes: "Just four or five years ago," says Richard Ray, administrative assistant to Sen. Nunn and a longtime prayer-group leader in the Senate, "I think I would have been laughed at" for expressing his personal faith.

The new chaplain of the Senate, Dr. Richard Halverson -- an evangelical who came in with the new Republican majority -- says that evangelicals, who once "tended to be in the closet," have "come more and more to the fore" since the Bicentennial as they rediscovered "the spiritual roots of the nation." In the Senate, he says, small-group activity has grown more evangelical since the 1980 elections "because of the influence of the Moral Majority. Some of the people elected are here because of the influence of conservative evangelicals."

Some view this trend with alarm. Christian fundamentalist conservatives "want to proselytize the whole country," says Sen. Lowell Weicker (R-Conn.), an outspoken opponent of the school-prayer action. "That's what I'm fighting against." And one esteemed local cleric who has been monitoring Congress' spiritual life for years says that "what troubles people in the classical Christian community is that" aggressive evangelicals "could become the more or less official religion of Capitol Hill because of their pertinacity."

Some even regard the weekly senators' breakfast as a de facto religious lobby. Halverson says there isn't the "slightest threat" because the participants hold radically different political views. Hatfield, who has been a member long enough to watch the attendance increase from 10 to its present 2 dozen -- and who opposes school prayer, among some other sensitive Christian issues -- says the group has a "diversity which is to me the whole essence of the Judeo-Christian faith."

Among the familiar supporters of pro-Christian legislation, Sen. Jesse Helms (R-N.C.) attends only infrequently at the Wednesday meetings, and Sen. Ernest Hollings (D-S.C.) does not go at all. And of those who do attend, former senator George McGovern says that "those guys have such a personal view of religion that it isn't reflected on the Senate floor -- if anything, they lean over backwards to avoid social issues." In fact, says McGovern, a minister's son who attended several of the meetings, "one of my criticisms is that they don't see the social implication of moral and religious faith."

But there are pressures. Hatfield says that certain stands are expected of him by Moral Majority. "A lot of their followers have so bought up this conservative religious dogma" that they "want to apply it across the board" to political positions as well. "It just doesn't follow," says Hatfield. "I reject that."

And Weicker, who has "absolutely no feelings at all" about the breakfast prayer group or "whatever anybody does on their own time," says he has seen a definite increase in the kind of Christian-oriented legislation exemplified by the "voluntary prayer" rider on the Justice Department appropriations bill. "The whole conservative movement has completely emasculated the appropriations process" by intruding religious issues, Weicker says. "Here we are, confronted with just a huge volume of pressing problems in the economy and international relations," yet senators are "literally volunteering to go over the same ground school prayer, busing that has been agonized over years ago."

The reason that such issues have been postponed until next year, says Robert Dugan Jr., director of the National Association of Evangelicals, is "partly a strategic maneuver -- holding off purposely until election year," when pressure on campaigning congressmen will be heaviest. "If nothing happens in the first two years" of the Reagan presidency, Dugan says, "you're going to see a great disillusionment."

Armstrong told a meeting of the NAE in 1979 that although "there are no Christian issues" before Congress, "there are some issues in which it's hard to see how a Christian could vote any other way." Prayer at Breakfast

It is 8 a.m. in the Dirksen Senate Office Building, and the monthly breakfast gathering of the Senate Staff Prayer Fellowship is about to begin. Each of the nearly 100 neatly dressed participants pays $1 at the door, one-fourth of what the Senate restaurant charges for the scrambled eggs, bacon and orange juice. (Contributions from Christian businessmen make up the difference.) Some are in their early twenties, some 40 years older. Each takes a name tag and a seat at one of five long tables.

After an invocation, a children's choir sings, new members stand and give their names (each participant is asked to bring a friend next time), and the speaker is introduced. Next month it will be Claire Schweiker, wife of the Health and Human Services secretary; in February former Nixon aide Chuck Colson. This month, it is Herbert E. Ellingwood, deputy counsel to the president, who is introduced as an active evangelical who has spoken abroad to audiences of thousands.

The crowd is already smiling when he takes the podium, but Ellingwood warms them up with the old Jesus-and-Moses-playing-golf joke (Bystander: "Who does he think he is, Jesus Christ?" Moses: "No, he thinks he's Arnold Palmer!"), offers a personal testimony of sin and forgiveness, warns them of the evils of "pornography" and enjoins the group to use their faith to become better government workers.

Afterward, Gina Bessey, a legislative assistant to Sen. Jepsen, shows a videotape report from the Christian Broadcasting Network that presents a 40-year history of Supreme Court rulings on separation of church and state. At each decision, the Supreme Court building grows slightly more red, until by 1980 it glows with a vicious crimson. The viewers bend forward on their chairs, and there are frequent sighs of "Ohhh!" or "Can you believe it?" The tape was made before the Dec. 8 ruling that universities may not prevent religious groups from meeting in state-financed facilities.

The group was started six years ago by Richard Ray, and has grown over the years: in part because of such speakers as Dallas Cowboys' coach Tom Landry, Chuck Colson and Rep. James Wright; in part because of Ray's subsidy plan. Once when he was meeting with a business group in Georgia, he mentioned that attendance at the meetings "might be cut down because a lot of people just could not afford to dig out $4 for breakfast." A bus company gave him his first contribution of $600, he recruited a few more, and soon organizations like the Kroger company were asking if they could help.

Ray, 54, is leaving soon to work for a private defense consulting firm downtown after nine years in the Senate. He runs a tight office ("no obscenity, no profanity, I'm not gonna tolerate it") and meets weekly with some other administrative assistants and Tom Barrett from Christian Embassy. Ray has never met with the Moral Majority: "I'd be glad to listen to them, but they've never asked." Meanwhile, he says, membership in groups is growing, especially among young staff members. "They have a lot of problems. They find themselves wondering why they're here, and they're lonely in a new environment."

Bessey, 25, the secretary-treasurer of the prayer fellowship, which can draw crowds of nearly 200 (for Colson, she says, "We'll have to get a bigger room"), came to the Hill in 1978. Two weeks later she met Linda Barrett of Christian Embassy, and became active in prayer meetings. A slender, attractive woman with a chipper sociability, she has started a special singles Bible-study group for Senate staff and attends a Thursday women's group. She says that believing in God "can make anybody who wants to be a more effective employe." Sometimes, she says, "when I'm in a dilemma, and I have 20 things to do in a few seconds, I say, 'God, just give me peace.' And He always helps."

In the Dirksen cafeteria, she sits beside the Barretts and another member of the prayer breakfast leadership, Gene Hammel, 40, executive assistant to Sen. Chiles. Against the nattering din of the other diners, they seem at deep peace, with broad, placid smiles.

Hammel attends yet another Bible-study group on Thursdays. He found his faith in December 1979, at a lunch meeting where Sen. Armstrong was speaking. Before then, Hammel had been concentrating on "the things of this world." But at the meeting, he says, "I wanted to learn to have a personal relationship with Christ, and I invited Jesus into my life."

Neither Bessey the Republican nor Hammel the Democrat believes that the religious activities have any political impact. "That's one thing we don't do," says Hammel. "We get enough of that the rest of the day," and attend such meetings for "fresh air." Bessey says that the six-year history of the monthly Wednesday breakfast shows its nonpartisan nature: "You see administrations change, senators and staffs change, but you never see God change." Over the past few years, she says, the number of groups has grown, largely through word of mouth and postings on Senate bulletin boards. "We would be thrilled if we could get every staff member," says Hammel. "That's our ultimate goal."

Over the table top he offers a thin, dogeared booklet about the size of a book of stamps. "I want you to have this," he says. It is titled "Have You Heard of the Four Spiritual Laws?" and copyrighted by Campus Crusade for Christ International. "Just as there are physical laws that govern the physical universe," it says, "so are there spiritual laws which govern your relationship with God." In 14 pages, the booklet explains that "God loves you and offers a wonderful plan for your life," but man is separated from God by sinful, "independent self-will" and can only be redeemed through Jesus Christ, whose spirit one receives "by personal invitation," and who will "control and empower your daily life and witness."

The booklet illustrates the relationship between "fact (God and His word)," faith and feeling with a diagram of a train in which "fact" is the engine and "feeling" ("the result of our faith and obedience") is the caboose. "The train will run with or without the caboose. However, it would be useless to attempt to pull the train by the caboose." A hundred million copies have been printed, according to Campus Crusade, whose president, the Rev. Bill Bright, has pledged to raise $1 billion by 1982 in "the most extensive Christian social and evangelization mission in recorded history."

Some find the ubiquitous pamphlet simplistic: "Ah, yes -- the 'four spiritual laws,' " says one evangelical senator, sighing heavily. "I wish the Lord had made things that clear while He was on earth." The Campus Crusade

"What we are doing in the booklet is reducing one essential part of the New Testament," says Rodney "Swede" Anderson, 43, head of Christian Embassy, the Washington arm of Campus Crusade for Christ, "the part about taking that one essential step toward Christ" in their larger mandate "to share Christ with people on all levels of society."

Anderson will not discuss the scope of his activity on the Hill ("Why do you want to know that?"), the frequency of his contact with legislators ("not unless you also ask that of the pastors of churches") or the senators he sees most often (some "might be sensitive to their names appearing").

The organization was created in the mid-'70s "to minister to the spiritual needs of the political, military and diplomatic communities," and now has a full-time staff of 23. In addition to five in its Arlington office and five on the Hill, Christian Embassy has two people full time at the United Nations, and the remainder ministering to the Pentagon and executive branch -- all on a yearly budget of $125,000, Anderson says. It started with "relatively informal types of gatherings," and then "if people are responsive and motivated, we try to spend some time with them personally."

"They think of themselves as being the representatives of the Kingdom of Christ in the nation," says Halverson. "They're the most aggressive evangelical organization I know." Anderson concedes that the group "takes the initiative" in the "how-tos of experiential Christianity": encouraging meetings, contacting new staff members, only rarely meeting with children of incoming congressmen. "That would be great to do," says Anderson, but he doesn't have a staff member accustomed to working with high-school students.

One Christian senator calls them "an exclusivist group" with a "systematized effort" that is "very oriented to political conservatism." Anderson says that, unlike Moral Majority, his group has no political interest, although "it is not our role to criticize" those who do. But "I have yet to be asked advice on a political question," says Anderson. Besides, "you are not going to find a monolithic Christian view of what's right." But Rev. Bright has spoken openly of his desire "to help bring Christ back into government." Isn't Christian Embassy's brand of evangelism bound to have some political effect? Only marginally, Anderson says: "If a person has come to Christ and is already concerned with society, it is going to have an effect on his attitudes." Faith and Politics

That's certainly true of Sojourners, a "radical evangelical" group that believes "faith has political meaning." The name applies to the church and "community" of believers centered at 14th and Euclid streets and to the magazine with a national circulation of 50,000. Both are headed by Jim Wallis, 34, who splits his time between practical "survival issues" for the poor, including housing, day-care and food programs ("the Bible is much more radical than Marx ever was") and liberal political concerns. "I can't preach the gospel without talking about the arms race and the gulf between rich and poor," says Wallis in the shabby but tidy magazine office at 13th and L streets.

Wallis, whose "main mentor" was Dorothy Day of The Catholic Worker, says "we are not a lobby group," but it organizes discussions around the nation and confers frequently with interested congressmen. One House member, facing a major foreign-policy speech, "called me up and said, 'I need some help. How should a Christian think on these things? What is the biblical approach to foreign policy?" He has dealt similarly with senators, whom he will not name, and is friendly with Sen. Hatfield. "We have talked for years about these things." Hatfield is a contributing editor to Sojourners Magazine.

Among other groups active in the Senate is the privately funded Fellowship Foundation, successor to the now-defunct International Christian Leadership. The group is best known for sponsoring the annual National Prayer Breakfast, addressed by the president and attended by thousands, and for prominent members such as Sen. Hatfield, Chuck Colson and former Sen. Harold Hughes, who calls it a "broad-brush," "non-denominational" effort to "change the lives of individuals." "They want to pray with people in power," says Wallis of Sojourners, who also does. "But I want to talk issues. For them, that kind of stuff is left at the door."

The group encourages a wide range of evangelical activities here and abroad, principally "discipleship" work with the poor, inner-city youths, American Indians and prisoners. "We try to be nearly invisible," says the Rev. Halverson, a Presbyterian pastor for 40 years -- most recently in Bethesda -- and a Fellowship leader who has been active in the prayer-breakfast movement since the '50s. "We have studiously avoided institutionalizing our movement," conduct "no financial campaigns" and "we guard our mailing list like a hawk." The membership "no longer wants any publicity," Halverson says; and Foundation director Doug Coe would not discuss the institution.

The group functions as a "switchboard" for a network of Christian government leaders in more than 30 countries in Europe, Latin America, Asia and Africa, Hatfield says. When he travels, he meets with his counterparts abroad; and when Christian leaders visit here, they stay at a special Fellowship Foundation guest house in Arlington called The Cedars, or visit Fellowship House near the Shoreham Hotel. "The idea," says Halverson, "was to have a place where people in public life could meet privately" to share their faith. The Christian Politician

Senators are often reluctant to admit that they belong to such groups. "Members of Congress are obviously leery of being identified with any enterprise that night embarrass them" in some political context, says Dugan of the National Association of Evangelicals.

Sen. Hatfield can understand that: "Oregon has the lowest church attendance of any state, and I have gotten some flak" for his religious views. "But after 30 years, they've grown used to me," and now "I get more criticism from within the Christian community for my political views."

"Every time a politician is kind of up front about his faith," Halverson says, "most people hear it as exploiting his faith for religious purposes." A former senator who observed the prayer-meeting phenomenon believes that "when it gets organized that way, I've always been suspicious" that religion is "an image created for political effect."

Not for Sen. William Armstrong, whose born-again experience in the mid-'70s was "perceived back in Colorado as my potential retirement from political life." His political advisers were horrified, he says: "They expected me to get Potomac fever, not to become a religious fanatic."

Armstrong "accepted Christ as my savior in a dining room" six years ago after "a fellow who I scarcely knew," a dentist, "presented to me the essence of the New Testament in a little pamphlet," the Campus Crusade's "Four Spiritual Laws." Armstrong, who was elected to the House in 1973 and the Senate in 1978, says, "The Lord prepares everyone for life-changing experiences in different ways. St. Paul was sent on the road to Damascus. Chuck Colson was given a term in prison. I was given a term in Congress."

He had been a "nominal Christian" ("frankly the kind of stuff that goes on at the prayer breakfast would have seemed a little freaky to me"), a successful businessman and popular congressman who by the age of 35 "had achieved everything I'd dreamed about." But instead of finding that fulfilling, he discovered "the essential emptiness of prominence."

But Armstrong says that his faith has not only improved his personal and family life, but "invests the work I'm doing here with a seriousness of purpose it couldn't have had otherwise. Not only am I accountable to my constituents, but I am accountable to the Lord for my faithfulness to those people." Now he "speaks on spiritual topics all the time . . . on airplanes, on street corners, in hotels or the floor of the Senate."

"If it hasn't happened to you yet," it probably will, Armstrong believes. The born-again experience has become "absolutely commonplace among thoughtful people who have contemplated the New Testament seriously."