Rep. Bill Nelson, a Florida Democrat with a conservative bent and an air of clean-cut earnestness, voted for Ronald Reagan's budget and tax packages last year. Today, he says flatly, "We overdid it on the tax cut." As for more budget cuts in social programs, "The president's not going to have my vote."

Surging deficits, high unemployment and stubborn high interest rates have soured the support that Reagan enjoyed among his ideological allies in the opposition party. The "boll weevils," that group of mostly Southern Democrats who swarmed to enact Reaganomics in 1981 are going off in different directions today.

Without their backing, the administration cannot hope to pass a new budget in the Democratic House without major changes. But when the president gave his State of the Union speech to Congress last month, most of the conservative Democrats were noticeably unenthusiastic, sitting on their hands while Republicans on the other side of the aisle applauded loudly.

In their first meeting of the year recently, some of the 47 members of the Conservative Democratic Forum, as the weevils formally call themselves, were talking about deferring the July, l983, tax cut and slowing down Reagan's projected defense spending increases--suggestions that would have seemed politically suicidal to many of them last year.

"It is imperative that we use the same scalpel on defense as on every other budget," said Charles Stenholm (D-Tex.), one of the group's founders. G. V. (Sonny) Montgomery (D-Miss.), chairman of the Veterans Affairs' Committee and a celebrated Vietnam-era hawk, is looking into ways of scaling back Reagan's defense increases. "If the president recommends a 7 to 8 percent increase, you could drop that to 4 or 5 percent and save a lot of money," he said.

"I can't believe these deficits," said Buddy Roemer, a freshman Democrat from Louisiana who supported the president's tax bill. Another Louisianan, Jerry Huckaby, says unemployment, up to 20 percent in some parts of his timber-growing district, is "as high as it's been and as tough as it's been in my lifetime. The question is how long people are going to keep the faith in the president until things get better."

It is a measure of how serious things are that Huckaby said he would now reluctantly advocate a windfall profits tax on deregulated natural gas, an upopular way to raise revenue among Louisiana oilmen.

Huckaby, along with Roemer and Phil Gramm (D-Tex.) plan to present some budget options to the group next week. "Some of us are toying with ideas on how to get the deficit down to the neighborhood of $50 billion," he said. That would be a massive cut in view of current estimates that the fiscal 1982 budget deficit will top $109 billion, more than three times Reagan's original projection.

"We hope to have real influence on the bargaining," Huckaby said. "This year we're not going to be a group lined up to rubberstamp the president's proposals. We're still behind him, but we want to steer him on a somewhat different course."

Last year, Gramm cosponsored the president's budget, along with Republican Delbert Latta of Ohio, allowing Reagan to characterize his program as "bipartisan." Rep. Kent Hance (D-Tex.), another forum member, cosponsored the president's tax package, along with Republican Barber B. Conable Jr. of New York. In both instances, more than a score of Conservative Democratic Forum members gave the president his margin of victory.

The group is varied and unpredictable, however. It includes members who supported the president on all the key economic votes, and others who supported him on none. It includes Tom Bevill (D-Ala.), longtime champion of billion-dollar dams, who wants to increase the public works budget this year, and Roemer, who has sponsored legislation to cut Bevill's water resources budget by 25 percent.

Some conservative Democrats such as Stenholm and Montgomery would like to cut even deeper into domestic social programs than Reagan has proposed. Others think the president already has gone too far. "I don't think there'll be many votes to tear up any more of the safety net if the deficit is above $100 billion," said Wes Watkins (D-Okla.).

Watkins would like to roll back the 1983 personal income tax cut--to save up to $45 billion through 1984--and close several loopholes, including the one that allows corporations to buy and sell their tax breaks. Bevill and Nelson also advocate a tax-cut deferral. "Anything else and you're whistling Dixie if you think you're going to cut budget deficits," Nelson said.

Whatever the outcome, the politics of the budget promise to be far different this year than last. Whereas six months ago, all the speculation was as to which boll weevils might switch parties to give the GOP a majority in the House, now the conservative Democrats are talking about starting a political action committee to elect more conservative Democrats. Their cars sport "Thank Goodness for Boll Weevils" bumper stickers and they distribute buttons stamped with a picture of their namesake.

In the president's courting of Congress that began in earnest yesterday, the weevils will play hard to get. "Last year, I voted for the president's program, even when I had questions about it, because I thought he ought to have a chance," said Nelson, echoing the comments of other forum members. "This year, if I disagree with him, I'll vote against him." CAPTION: Picture, REP. JERRY HUCKABY . . . deficit should be nearer to $50 billion; Picture 2, REP. BILL NELSON . . . Congress overdid it on Reagan tax cut.; Picture 3, REP. BUDDY ROEMER . . . rethinking support of Reagan tax cut.; Picture 4, REP. CHARLES STENHOLM . . . cuts also needed in defense spending.