By Susan Milligan and Rick Klein, Globe Staff

WASHINGTON -- Moderate Republicans in the House of Representatives, once ignored or punished for their fights against weakening environmental laws and a conservative social agenda, have found new power at a time when GOP leaders are struggling to keep the White House's legislative agenda on track.

Yesterday, House leaders pulled their $51 billion budget-cutting bill from consideration, amid a backlash from centrist Republicans upset about its impact on social programs like Medicaid, food stamps, and student loans. It was a stunning concession by House leaders who are used to getting their way with few impediments.

Even before the last-minute move to delay the budget, House leaders had yielded to centrists' demands and scuttled plans to open the environmentally sensitive Arctic National Wildlife Refuge in Alaska to oil and gas exploration. Earlier this year, the moderates moved ahead with their agenda in favor of stem cell research, and prevailed against President Bush's effort to water down wage and labor laws for contractors hiring workers to rebuild New Orleans and the Gulf Coast.

Sarah Chamberlain Resnick, executive director of the centrist Republican Main Street Partnership, said GOP leaders have awakened to the fact that the most valuable vote a House member can cast is for House speaker; if moderates are forced too far to the right, their constituents won't send them back to Washington.

''The reality is, we need to get reelected. I think the leaders get that" now, Resnick said.

Last year's highly contested presidential election ''mobilized a lot of people" who were concerned about the rightward trend the country appeared to be taking, said Representative Charles Bass, a six-term New Hampshire Republican who led the effort this week to get the Alaska oil-drilling proposal out of a budget-cutting bill.

''I think it gave a lot of life and vitality to the centrist movement in this country," said Bass, whose home state was the only one nationwide that switched from supporting Bush for president in 2000 to voting for Senator John F. Kerry, a Democrat from Massachusetts, in last year's election.

The stalled budget bill is the biggest leadership challenge for House Speaker J. Dennis Hastert and House majority whip Roy Blunt since Tom DeLay, the powerful Texas Republican and former majority leader, was indicted on a conspiracy charge involving campaign financing and forced to step down in late September. Conservative Republicans have threatened a leadership challenge against Blunt if he doesn't cut spending, while moderates balked at the cuts to programs being recommended by House leaders.

''The leadership is a little fragmented at this point, and they're dealing with both ends of the rainbow," said Representative Michael Castle, a moderate Republican from Delaware.

Democrats pounced on the budget vote delay, calling it a sign of a Republican party in turmoil. After Democrats' strong showings in state elections on Tuesday, Republicans are anxious about pursuing Bush's agenda, said House minority whip Steny Hoyer, Democrat of Maryland.

''It shows a Republican majority in disarray and bereft of ideas that address the needs of the American people," Hoyer said. ''The American public has expressed its opinion on the failed policies of this administration."

The budget-cutting bill has been stalled for three weeks now, and Blunt put yesterday's session into recess for five hours in a last-minute scramble for votes. But closed-door meetings with rank-and-file Republicans came up short of votes and the budget bill was delayed again until next week.

''We were not quite where we needed to be to go to the floor," said Blunt, a Missouri Republican, who predicted that he will get a majority behind the measure by the end of next week. ''We'll come back next week and continue to work with our members."

Moderate Republicans have long enjoyed influence in the smaller Senate, which has been more evenly divided between parties in recent years, and which gives greater power to its 100 members.

Indeed, a single moderate Republican -- Senator Olympia J. Snowe of Maine -- held up a tax-cut bill yesterday because her vote against it threatened to deadlock the Senate Finance Committee, which would have killed the measure. The bill will come up for a vote again next week.

But in the House -- particularly under DeLay's leadership -- moderates have been marginalized. Republican leaders have depended on a small band of conservative Democrats to keep their agenda moving on priorities that centrist Republicans have opposed. And on the toughest of votes, Republican moderates were bludgeoned into following the party line with threats of delayed committee assignments and sidetracked legislative priorities.

As Bush's popularity continues to slide and the House GOP leadership falters, party moderates have served notice: They won't be bullied into supporting bills or budget cuts they don't like. A group of about two dozen centrist Republicans raised questions about drilling in the Alaska wildlife preserve, a top issue for environmentalists.

Faced with tough budget decisions, moderate Republicans like veteran representatives Christopher Shays of Connecticut and Sherwood L. Boehlert of New York have doubts about cutting Medicaid and other social programs when people are struggling in a sputtering economy. Shays said such votes are even more difficult in the current environment, with the GOP hampered by ethical lapses, low approval ratings, and public outrage over high gas prices and the Iraq war.

''It's one thing to take a tough vote when everybody has their act together," Shays said. ''It's another thing to take a politically tough vote" when the public is questioning Washington's one-party rule.

The Democrats' Hoyer said GOP moderates had ''a pattern of rubber-stamping" GOP initiatives -- criticizing the conservative agenda, then ultimately voting for it -- but those days may be at an end. Democrats scored big wins in New Jersey, California, and Virginia on Tuesday, ''and that gives pause to the moderates in the Republican party," he said.

The House package would cut $51 billion from the budget over five years, largely from social programs such as federal student loans, child support enforcement programs, and food stamps. The Senate approved a version last week that would pare the federal budget by about $35 billion.

Fiscal conservatives, including moderates, want to rein in the deficit and the record-high national debt, though they're short on specifics. But some of the same lawmakers are wary of cutting programs that might hurt their constituents, especially right before an election year with the GOP's approval ratings at an ebb. (The three Republican House members from Connecticut and the two from New Hampshire are high on the Democrats' target list for 2006.)

House leaders are expected to take the matter up again when they return on Tuesday, and Democrats, while hopeful, acknowledged that Republicans could ultimately prevail.

GOP leaders in the past have kept votes open for hours, strong-arming recalcitrant Republicans to deliver votes on contentious bills like the Medicare prescription drug package and the recent energy bill.