In the heat of last summer, U.S. Sen. Paul Sarbanes (D-Md.) had the look of a marked man. The potent National Conservative Political Action Committee had named him its first liberal target for 1982, filling Washington and Maryland airwaves with expensive anti-Sarbanes advertisements of the sort that helped defeat four prominent liberals in 1980. And the normally reserved senator, a critic of long campaigns, was out on the stump a full 19 months before the election, complaining between speeches, "It's not fair."

But now, seven months and$250,000 into the "get Sarbanes" campaign, NCPAC's opening shot for 1982 appears to have become a blessing in disguise for the junior senator from Maryland.

Within his home state, it has galvanized Sarbanes' friends and divided his foes, drawing criticism from some prominent Republicans who say they dislike "outsiders" and "negativism" in state politics. And it has transformed the relatively unknown senator into something of a national cause not just for his opponents in the New Right but for Democratic and liberal fund-raisers who hope to stop NCPAC's march as well.

Sarbanes' 1982 campaign war chest, by his treasurer's account, topped the $250,000 mark late last month, almost as large as it was 10 days before the 1976 primary, and fund-raisers say they would not have collected so much so soon had it not been for the NCPAC assault. The AFL-CIO issued an early endorsement of Sarbanes, along with an attack on NCPAC, and more than 1,300 letters of support poured in from around the state and the country: "I hope you win, if only to show up those right-wing dingbats," wrote a woman from Kansas, who enclosed a check. Meanwhile, friends in several states called to offer to hold fund-raisers.

Republican Rep. Marjorie Holt, called by GOP leaders "the person most able to beat Paul Sarbanes" when she expressed interest in running last summer, backed off from the race last month. A top aide said privately that NCPAC created a backlash that "did Sarbanes more good than they did us."

The Maryland GOP has started its own direct-mail campaign against Sarbanes, pegged to his leadership early this year in opposing the confirmation of Secretary of State Alexander Haig. But the party raised little more than the cost of the mailing, according to state GOP chairman Allan Levey. He added that it did bring in volunteers to work against Sarbanes, and the national party has pledged $250,000 to help defeat the first-term Democrat, whom it calls "one of the most vulnerable" senators in 1982.

As for the NCPAC campaign in Maryland, Levey said, "I would rather we would have been able to do it ourselves. There's a general feeling that people don't like outsiders coming into the state."

The reaction to the NCPAC effort has delighted Democrats in national political circles. "It's increasingly clear that groups like NCPAC become less effective when you throw the spotlight on them," said Ann Lewis, political director of the Democratic National Committee. "NCPAC is like cockroaches. They operate better in the dark."

But NCPAC chairman John T. (Terry) Dolan contends that the campaign is working well. He said last week that the Virginia-based organization, boasting a mailing list of 350,000 potential conservative contributors, probably will spend even more than the $400,000 originally budgeted in Maryland, adding that NCPAC polls show a 4.4-point drop in Sarbanes' "favorable" rating and a 3.6-point climb in his "unfavorite" rating following the first advertising offensive last April. And he pointed out that election day is still a year away.

To those who note that Sarbanes has lots of money, Dolan warned: "So did McGovern," recalling NCPAC's offensive against former Sen. George McGovern (D-S.D.), one of 1980's liberal casualties. "The people of Maryland now view Sen. Sarbanes as more liberal, anti-Reagan and pro-Ted Kennedy," Dolan continued. "If they choose to reelect him, knowing his record, that's completely their prerogative. It's called democracy."

Sarbanes' liberal reputation comes largely from his 1974 role in the House of Representatives' impeachment hearings on then President Nixon and his leadership in Senate votes for the Panama Canal treaties and nuclear arms control, and his votes against the Reagan budget cuts. A Rhodes Scholar and a graduate of Princeton and Harvard Law School, he is known for studiousness, deliberation and working behind the scenes, a style that left him with low name recognition until the NCPAC ads began to increase it. Lately, he has reminded voters that his liberal record is also strongly on the side of "working people" and small business.

While Dolan contends Maryland voters are far more conservative than Sarbanes -- he scored 83 percent in the liberal Americans for Democratic Action ratings, and zero in the American Conservative Union's -- the senator for now appears to be wearing the anti-Reagan mantle well in his home state, one of the most Democratic in the country. He was repeatedly interrupted by applause last week at the Maryland Congress of Parents and Teachers convention, where he gave the keynote speech, attacking the Reagan cuts in education, and calling federal budget director David Stockman "a man who knows the cost of everything and the value of nothing."

The week before, he got a similar response from audiences of steelworkers and auto workers, when he attacked "trickle down" Reaganomics, asserting that the administration's policies help the rich and penalize working people.

"This the NCPAC attack hasn't worried me and it hasn't made me angry. It's made me determined," Sarbanes said in an interview after his PTA speech. "I think Marylanders look beyond any particular issues to read the quality of the work and judgment that comes from their senator . . . I'm not about to abandon the values that have been at the heart of my political life: belief in full employment, in education, in better health for our people. I believe those are the values that will claim a clear majority of the people of this state. If I'm proven wrong, then so be it."

So far, according to Sarbanes' aides, his mail says he is right. The letters started coming soon after NCPAC's newspaper ads appeared in Baltimore and Washington, headlined "Paul Sarbanes thinks you should be paying more taxes." The ads closed with a clip-out coupon to be mailed to the senator's office to "Let Paul Sarbanes know what you think about his dismal voting record."

While many of the coupons arrived intact, dozens came in from supporters who scrawled over certain words to make the coupons read as praise for Sarbanes' "excellent" voting record. One Silver Spring man penciled "BULL" across it and enclosed a contribution to Citizens for Sarbanes. Other letters came from Kansas, from California, from Florida, from New York and other states where articles about the NCPAC campaign were published.

Lt. Gov. Sam Bogley, a conservative Democrat who said he had considered challenging Sarbanes in the primary, has decided against it for now, saying he believes NCPAC "tainted" the atmosphere. The freshman senator's only declared Republican challenger is Prince George's County Executive Lawrence Hogan, for whom party leaders say they have little enthusiasm.

The attack also fired up Sarbanes' campaign staff, according to treasurer Charles Kerr. "We don't like it when a member of the family is attacked this way," said Kerr, a Baltimore lawyer who has worked actively for Sarbanes since his election to the House of Representatives in 1970.

And around the country, Sarbanes' traditional supporters in organized labor, in Greek-American communities (he is the first Greek-American elected to the U.S. Senate) and in the Democratic Party redoubled their efforts. Labor unions accounted for most of the $47,000 in contributions from political action committees that Sarbanes had received by the end of last month.

"He's one of our national heroes," said California State Sen. Nick Petris of Oakland, another Greek-American leader. "I read about the attack in the paper and I saw they were ganging up on him and I called him up. I said, 'Hey, Paul, we better get going.' And he said, 'We already are.' "

Petris, who is organizing two weekends of fund-raisers for Sarbanes in San Francisco and in Los Angeles, inviting Democrats around the state, predicted that Sarbanes' California supporters will raise "a lot more money" because of the incentive to send a message to NCPAC.

"We just figure forewarned is forearmed," he said. "And since we've had so much forewarning, we'd be foolish if we didn't arm ourselves."

The NCPAC attack on Sarbanes has now become fodder for a national direct-mail war between Democrats and liberals on one side and the New Right and Republicans on the other, with both sides saying that the fight is good for fund-raising.

Democrats for the '80s, a political action committee organized by establishment Democrats to try to revitalize the party following last year's debacle, made Sarbanes its first public cause, buying $20,000 in radio ads last spring to counter the NCPAC offensive. The group later cited its efforts on Sarbanes' behalf in a direct-mail solicitation, to demostrate one reason why Democrats should contribute generously to the new committee.

The Council for a Livable World, a national nuclear arms control lobby, has raised $12,500 for Sarbanes, according to a spokesman, through a national mailing that says in part: "Senator Sarbanes' quiet effectiveness is generally recognized, not least by the ultra-right NCPAC which selected him as their first target for 1982." Pro-PAC (Progressive Political Action Committee), which was formed to fight NCPAC, has also featured Sarbanes in advertisements.

Meanwhile, the NCPAC mailings continue, with the latest saying: "Sarbanes has opposed a strong national defense, tax cuts, and the President's Economic Recovery Program. And when we weren't looking, he voted himself a whopping 29 percent pay raise, liberal funds for abortion, busing, more bureaucracy and aid to the Communists in Nicaragua . . . . Sarbanes is more liberal than Ted Kennedy. Is that what you want?"

Back in Maryland, the airwaves are quiet for now. No ads have run since September, and NCPAC just let the lease run out on seven billboards in Baltimore. But phone banks are calling potential Sarbanes opponents around the state, and Dolan said more radio and television ads branding Sarbanes as a big-spending liberal will resume by early 1982.

"To be perfectly honest," he said, NCPAC targeted Sarbanes in part because he lives right next door to Washington, putting all 534 other members of Congress within earshot of the attacks on his voting record. The unspoken message is that other congressmen could be next. "We wanted to use it as an educational tool for other members of Congress," Dolan said.

But in Maryland, NCPAC's voter education project has not encouraged all conservative Republicans. An aide to Rep. Holt sized it up this way: "Yeah, they accomplished something. They reminded Marylanders that Paul Sarbanes is a liberal. Thanks a lot. Marylanders are liberals."