Not six months ago, Warren Rudman, the kinetic first-term senator from New Hampshire, was, he insists, a defeated and deflated politician, seriously contemplating a retirement into obscurity and political irrelevance.

"I had decided that, look, I don't like Washington," he said yesterday, with all the subtlety of an armored truck. "I don't mean to put this city down -- it's a wonderful city with wonderful people -- but there is no question when I left here in August, I thought I would not run for reelection . . .

"I don't socialize. I'm not particularly interested in spending evenings with a bunch of strangers. Quite frankly this is not a town that cares for Warren Rudman as a person, or for anything I stand for or anything I value. If the word 'senator' disappeared from in front of my name, my invitations would go from 200 a week to two a year.

"So, if you can't accomplish anything -- and we're here after five years of a Republican president and a Republican-controlled Senate and we're not getting anything done -- I figured: Hey, I don't particularly enjoy living here, and I'm not much for the social side of life down here, and I can't find a lot of things to my liking; I mean, what the hell?"

But next week, Rudman, 55, coauthor of the hottest piece of legislation since the 1981 Reagan tax cut, will tell his New Hampshire constituency that, well, he's had a change of heart. "Obviously, now I feel almost a moral obligation to get things done," he said, sitting in his Hart Building office as the 99th Congress began gearing up for what may be the bloodiest budget battle in U.S. history.

This week the comptroller general of the United States will tell the president what cuts he must make in the federal budget for fiscal year 1986, in accordance with the notorious Gramm-Rudman-Hollings Balanced Budget and Emergency Deficit Control Act. The final version of the legislation sets targets for enormous deficit reductions between 1986 and 1991. If Congress fails to meet the targets, and the administration fails to agree on the cuts, domestic and defense spending will be forcibly slashed $12 billion this year. And if Congress and the president can't agree on reductions for FY '87, another $50 billion will be automatically cut -- one month before the 1986 election.

"October 1986 will be like the last two minutes of the Super Bowl with the score tied," Rudman says. "That's when the deals will be made."

These are wild times for Warren Rudman. Senators are weeping to him over program cuts in their states. Special interests are knocking at his door. The Pentagon! The schoolteachers! The drug enforcers! And the news media just can't seem to get enough of Rudman, Hollings and Gramm.

"The big change in my life has been the media attention," he says. "Last week I went way out of my way to do 'MacNeil-Lehrer' when I was visiting my mother and father in Florida. I had to travel to some little studio in the middle of nowhere to do a satellite remote feed."

Yesterday, Rudman was acting like a man more ready for war than for acquiescence -- quoting Lincoln, producing his favorite news story on his bill, and basically dismissing any criticism of the legislation that made his a household name. The former New Hampshire attorney general speaks in New England workingman's cadences, sounding like a toned-down version of Tip O'Neill.

His home-state constituency and his mail overwhelmingly support the bill, he says; Congress voted it into law last month; the president signed it. Still, support for the measure has been rather wimpy.

"It's a bad idea whose time has come," Rudman enjoys saying.

Among other things, critics of Gramm-Rudman-Hollings call it the desperate act of a government admitting it is incapable of governing itself.

"On the contrary," the senator argues, "it's saying we can govern ourselves. On the contrary, it's saying that the American Congress finally said 'Enough -- we are not going to continue financing our future through deficits.' I think this is a strong signal that we have finally taken the bit in our mouths."

Rudman's biggest battle, though, has been with Republican hawks who claim that his bill is tantamount to laying down in front of the Soviets and that it will be a prelude to World War III.

When a Pentagon spokesman told The Washington Post that passage of the bill would send "a message of comfort to the Soviet Union," Rudman went nuts. He savaged the spokesman on the Senate floor, then called Defense Secretary Caspar Weinberger.

"If The Washington Post printed profanity I'd tell you what I think of that," he says. "The Russians ought to be dancing in the street when they see this country spending itself into bankruptcy. They can defeat us without ever firing a shot. That is foolishness. Secretary Weinberger told me that statement was not authorized . . . It was an asinine statement, an asinine statement. I just told him, 'I think your department ought to watch what it says.'

"One thing I learned a long time ago: I don't mince words with anybody."

Rudman says that while he'll run again, his life here won't change. His wife will remain in New Hampshire and he'll continue to avoid what he calls the glitz of Washington life.

"I have to tell you something, and I'll leave you with this. I'm a temporary occupant of this place. The Senate seat that I occupy belongs to the people of New Hampshire. I'll occupy it for some period of time and then I'll be gone. I don't want to get too comfortable here.

"There's gotta be a life after this place."