It was a Saturday night in the middle of August, and John Claggett Danforth fairly burned with righteousness.

The Republican senator from Missouri had been a vocal supporter of the 1986 tax overhaul bill. He had helped write it as a member of the "core group" of the Senate Finance Committee and had served on the conference committee that fused the House and Senate versions. But now, in a surprising turnabout as the joint conferees prepared to vote, Danforth was in a House committee room delivering an impassioned stemwinder against the result.

"It was produced behind closed doors by the chairmen of two committees," he charged in a swipe at Sen. Robert Packwood (R-Ore.), chairman of Senate Finance, and Rep. Dan Rostenkowski (D-Ill.), head of House Ways and Means. "Now we're getting the bum's rush to approve this bill, because if we don't -- why, people will find out what's in it."

He rose a little out of his chair to slam his fist on the table. There was more brimstone here than Danforth, an ordained Episcopal priest, would ever employ in a homily.

"When you cross a fair maiden, the Senate bill, with a gorilla," he added, "you get a gorilla."

The dark-suited spectators erupted in applause. People at home must have thought they were watching a Frank Capra movie -- no matter that the cheering came mostly from lobbyists who were about to lose their loopholes. In the end the conference report was overwhelmingly adopted, and the lonely dissenter hastened from the room.

"He doesn't flare up that much, but he was really mad," said Sally Danforth, his wife of 29 years. "My mother called and said, 'I've never seen Jack so worked up.' When he came home he was still mad. I told him, 'I'm sure glad you don't treat me that way.' "

As the Senate's leading opponent of the tax bill -- which passed the House Thursday 292 to 136 and was expected to meet with similar success as it was taken up yesterday by the Senate -- Danforth has become the champion of a hopeless cause.

"I think it's unlikely I'll be able to do anything," he said recently in his Russell Building office, this time speaking with his customary calm. "I think that what happens in Congress, when something is a fait accompli, people are very reluctant to tilt at windmills. And I think people do feel that this might be quixotic enterprise. But my view is that in another five months or a year, a vote against the tax bill is going to look very good, and a vote for the tax bill is going to look very bad."

Danforth was looking very long and lean, as befits a grandson and heir of the late William H. Danforth -- author of the inspirational motto "Stand tall, think tall, smile tall, live tall" and founder of the Ralston Purina Co.

Occasionally Danforth, 50, one of the richest men in the Senate as a major stockholder in the huge pet food concern, has been referred to in print as "Dog Chow Jack" -- a source of minimal amusement. "I've never heard anybody call me that," he replied icily when asked about the name, then grinned. "But people have called me worse that that."

When a visitor spilled coffee on his richly patterned rug, the senator became the soul of good humor. "This is an oriental rug -- at least the rug salesman who sold it to me told me it was," quipped Danforth, who is a powerful foe of Japanese trade practices and, as chairman of the Senate Finance subcommittee on international trade, has often threatened to restrict Japanese imports to the United States.

Now he has positioned himself as the most visible -- and at 6 feet 3 1/2, the tallest -- of a small coterie of senators, including Lowell Weicker (R-Conn.)., Carl Levin (D-Mich.), Larry Pressler (R-S.D.) and William Roth (R-Del.), who have come out against the tax bill, which reduces shelters, lowers marginal rates and shifts the tax burden from individuals to corporations.

As for the bill's effect on Danforth's finances, "I haven't computed it," he said. "My guess is that I'd be a winner unless I had a lot of capital gains, because those taxes are going up. In the typical year -- not the unusual year, but in the typical year -- the rate cuts would have to benefit me because I'm not in a lot of shelters ... I'm not saying everything about the bill is bad. I think getting rid of shelters is good."

In recent weeks he's been making speeches on the floor, issuing press releases and writing "Dear Colleague" letters condemning the bill, which President Reagan backs strongly, as antigrowth, antijobs, antifuture, bad for capital investment, charities, research and development, American competitiveness in world markets and the solvency of the federal budget -- that is, just plain bad for the country.

Yesterday Danforth was at it again, calling the bill "a serious blow to the economy of the United States" in a floor speech as the debate got under way. "It will have profound repercussions, and it leads America not in the direction of greater strength but greater weakness." He seemed to be speaking more in sorrow than in anger.

"I think all of us enjoy hearing him talking in the committee in the conference and elsewhere," said tax conferee Sen. John Chafee (R-R.I.), who first met Danforth when they were serving as trustees of Yale University in the early '70s. "He's not a preacher or somebody who's a goody-goody two shoes. It's not that he's lecturing members or thinking he's more righteous. Although I've never heard him swear, I suppose."

"Anytime anybody punches the tax bill button, you can be prepared for the spiel," Danforth said in his office. "Here I am, somehow with a spotlight on me relating to the tax bill, and what am I gonna say? ... Do we say to the American people, you know, 'There's good news and bad news'? The bad news is that we have a $ 2 trillion national debt. The good news is that your responsibility for paying for this load is going to be reduced. Is that the message that always comes out of Washington? Eat, drink and be merry? Live for today? ...

"I got companies in Missouri that are thrilled with this bill. Businesses that are delighted. But I think there is a strong underlying feeling on the part of the American people that this live-for-the-moment routine is wrong. I think there is a strong skepticism that the national debt is $ 2 trillion and their taxes are gonna be reduced by five bucks a week ... It's the politics of joy, to use [Hubert] Humphrey's phrase. To me that's a policy without a future, and someday we're going to have to face up to that -- and I just want to help people face up to that."

Others, however, have ascribed to Danforth a somewhat baser motive. They say he was comparatively quiet in the joint conference sessions, sticking by Chairman Packwood, until his colleagues began slicing away at an $ 8 billion tax break for defense firms. Known as "the completed contract method of accounting," it allows long-term contractors -- not least of which is McDonnell Douglas Corp., the largest private employer in Missouri -- to defer paying income taxes until a contract is completed, sometimes indefinitely. The current bill reduces the break to $ 4.5 billion, and Danforth has mentioned, almost in passing, that it will cost McDonnell Douglas hundreds of millions of dollars.

"All it is," said a Senate aide of Danforth's opposition, "is bread-and-butter constituent politics, with a philosophical cushion."

"That's not the way I view it at all," Danforth said in response. "I had very great concerns about the bill as we were proceeding, at a time when I thought we were going to do much better on the completed contract method of accounting than we ended up doing." At one point in the negotiations, as the Senate bill seemed to be taking on more and more baggage from the House bill, "I was quoted as saying that was I gagging and choking and rolling around on the floor.

"And I went to the next meeting with the House conferees and said at that time that this bill is getting increasingly difficult, and if we keep moving in this direction I will oppose it -- and maybe even have to filibuster it."

But more often than not, Danforth kept his own counsel.

"You don't want to jump ship prematurely," he said, "because as soon as you jump ship all your bags are tossed overboard after you. After the bags were thrown out anyhow, I was more than willing to rush to the rail and take the plunge."

"I like the idea of tax reform," said Sally Danforth, settling into a sofa the other day in the Danforths' lushly appointed colonial in Northwest Washington. "Lately I've been hearing about it over and over again. It got to be where I thought if I ever heard about the tax bill again, I'd die. But Jack was a tax lawyer in New York and used to carry around a big book on the tax code. It's not that it's boring, it's just not my interest."

They have five children, aged 14 to 26, one of whom could be heard yodeling elsewhere in the house. "We've learned to tell the difference between country-western and heavy metal," Sally said, without volunteering to categorize the music at hand. The Danforths have been known to grace the family car with a bumper sticker saying "Honk if You Love Willie Nelson."

Earlier this month Danforth turned 50. "It felt fine, about the same as turning 49, only more so," he said. "I didn't go back to bed and pull the sheets over my head."

"He's the funniest man, and no one knows it," Sally Danforth said. "He has a dry wit and this great stone face, and a lot of people don't realize he's being funny. They think he's aloof. I guess because he's tall."

A fly fisherman and horseman who likes to repair to his 900-acre farm in Clayton, Mo., Danforth reads Trollope insatiably and, according to his wife, "hangs out at Erol's" when in Washington. And he spends time instilling in his children the values of noblesse oblige with which he was raised in St. Louis. "My own self, my very best," he is moved to repeat on occasion. Each Danforth child, on graduating from high school, receives a gold medallion inscribed: "Aspire nobly, adventure daringly, serve humbly."

"I wanted to be a senator when I was quite young," Danforth said. "I was interested in politics, and I thought the Senate was the highest calling. My parents took me to Washington, and I thought, 'Gee, this looks interesting.' And I went to the Republican National Convention when I was 15. I have the most interesting job. I like it a lot -- so I'm very happy."

He was first elected to the Senate in 1976 after an unsuccessful try in 1970. Considering a political career that spans two decades, including a tour as attorney general of Missouri, it is ironic that Danforth has little love for the tumult of campaigns. "Sometimes I think, 'What am I doing this for?' " he said.

The 1982 race against Harriett Woods, this year's Democratic nominee to replace the retiring Sen. Thomas Eagleton (D-Mo.), "was a nightmare," said Sally Danforth. "We thought it ws going to be a nice soft campaign, and here was this woman with enormous boxing gloves." Indeed, Sally recalled, at one campaign stop "somebody punched me in the stomach."

"Before that, Jack was much more laid back -- a much softer, innocent, gentler person," she said. "Now he's much tougher. There's no question that now he has a thicker skin."

Danforth was a religion major at Princeton, and took degrees in both law and divinity at Yale. He became a minister out of an intellectual attraction, particularly to the practical theology of Reinhold Niebuhr, rather than out of an intense spiritual experience, he said. When he was a lawyer in New York in the mid-'60s, he doubled as an assistant chaplain at Sloan-Kettering Institute for Cancer Research, ministering to terminal patients. "People were dying all the time," Sally recalled. "We learned about the way they died, the way they handled death."

Today Danforth officiates at morning services most Tuesdays at St. Albans Chapel. But to hear him tell it, his role as a priest doesn't carry over into his portfolio in the Senate. "It's just a part of my life," he said. "I'm ordained, and it's important to me to exercise it. But it would be far-fetched to see it in any obvious relationship to my job here."

He is not particularly interested in the presidential candidacy of fellow Yale Law School graduate and minister Pat Robertson. "I'm not sure I've followed all the nuances of it. I don't anticipate signing on to his campaign," Danforth said. "I think we have a very diverse, pluralistic country, and it's important for people in elected office to make it very clear that you're elected to serve all the people and that you're not operating in some sectarian way."

Asked his ambitions, Danforth was vague.

"I don't know," he said, "I'm in the Senate now. I'd like to be reelected. And someday I'll be leaving public life and going home, hopefully on my own steam, not to be ridden out on a rail. I guess it's conceivable that I'd go into the ministry full time, but I'd kind of doubt I'd do that."

Perhaps the ministry has given him a contemplative mien. For all his passion against the tax bill, Danforth took a horseback-riding vacation in Wyoming immediately after his defeat in the conference committee.

"There wasn't anything to be done by staying around," he said. "I made a bunch of calls at the beginning of that recess, talked to maybe 10, 12 senators, asking them their views, and they were all keeping their powder dry. They were all saying, 'Gee, we don't know, we'll have to see' -- so it was clear that it was the beginning of a process of fermentation that would take a little while to develop."

And although he threatened it, Danforth quickly gave up the idea of a filibuster. "Well, I'm not even sure we'd get 40 votes," he said, referring to the number necessary to keep a filibuster going. "I do think that after more than a year's effort Congress should work its will. And we have so darn many things to do around here that we've just got to get on with them."