Grover Norquist, president of Americans for Tax Reform (ATR), speaks at the Bloomberg Washington Summit on May 1 in Washington. (Joshua Roberts/Bloomberg)

In GOP activist circles it is known simply as “the pledge,” and over the past generation it has become the essential conservative credential for Republicans seeking elective office. Of the 242 Republicans in the House today, all but six have signed the pledge.

But now, an increasing number of GOP candidates for Congress are declining to sign the promise to oppose any tax increase, a small sign that could signal a big shift in Republican politics on taxes.

Of the 25 candidates this year promoted by the National Republican Congressional Committee as “Young Guns” and “Contenders” — the top rungs of a program that highlights promising candidates who are challenging Democrats or running in open seats — at least a third have indicated they do not plan to sign the pledge authored by anti-tax crusader Grover Norquist.

Two of the seven candidates promoted by the NRCC as the “Young Gun Vanguard” — candidates competing in open seats that are considered Republican-leaning — also have declined to sign.

The pledge pushed by Norquist, president of Americans for Tax Reform, compels candidates to “resist any effort” to raise tax rates for individuals and businesses. Signers also pledge to oppose the elimination of tax credits and deductions unless they are matched dollar-for-dollar with tax cuts.

Norquist insists ATR, the powerful group he founded in 1986, is ahead of schedule in collecting pledge signatures from congressional candidates for the year. He points out that the likely GOP nominee for president, Mitt Romney, has signed the pledge. But there have been defections.

Republican candidates declining to sign generally indicate that they nevertheless oppose tax hikes. But some chafe against the constraint on eliminating tax loopholes, believing those restrictions limit Republicans’ ability to negotiate seriously with Democrats on a deal to tackle the nation’s mounting debt.

In Pennsylvania, Republican state Rep. Scott Perry said he was disappointed to see his party’s presidential candidates — all but one of whom signed the pledge — uniformly indicate in a debate last year that they would reject a deficit reduction deal that paired $1 in revenue increases for every $10 in spending cuts.

“I just think it’s imprudent to hem yourself in where you can’t make a good agreement that overall supports the things you want to do,” said Perry, who said he generally opposes tax increases but recently won a Republican primary in a conservative district over candidates who had signed the pledge. “I just don’t see what the point of signing would be for me. . . . I’ve got a record, and everyone who wants to know where I’ve been and where I’m at can look to that.”

Others insist they can make promises to voters without signing a pledge circulated by a Washington lobbying group. That indicates Democrats might be having some success at painting Norquist as a D.C. insider instead of the anti-establishment rebel he portrays himself to be.

“I don’t want to get tied up in knots,” said Richard Tisei, an NRCC Young Gun and former Republican state senator in Massachusetts who is running against Democratic Rep. John F. Tierney. “If there’s a loophole that can be closed that ends up generating additional revenue that can be used specifically to pay down the national debt, I’m not going to lose sleep. And I don’t want to be bound by the pledge not to close it.”

The refusals among some new candidates come as a handful of incumbent Republicans who signed the pledge when they first ran for office also are publicly rejecting it.

Freshman Rep. Scott Rigell (R-Va.), who signed the pledge in 2010, recently posted an open letter to constituents indicating that he would not renew the promise as he runs for reelection. He said he fears it could stand in the way of an everything-on-the-table approach to tackling the mounting debt.

“Averting bankruptcy requires us to grasp the severity of our fiscal condition and summon the courage to speak boldly about the difficult steps needed to increase revenues and sharply decrease spending,” he wrote.

An erosion of support among candidates would be especially significant because Norquist has long aimed to collect signatures from Republicans before they take office. He encourages candidates to use their pledges to help to define their tax stance for voters.

Once the pledge is signed, Norquist considers it binding for the remainder of the candidate’s career in public service if he or she wins office.

In an interview, Norquist said the pledge is a strong as ever. He noted that in the pressure-cooker days of the debt-ceiling debate last summer, Republicans held firm against tax increases and wrested a deal from Democrats to lower deficits through spending cuts alone.

“That was when the pledge was tested and the commitment of Republicans not to raise taxes was really pushed hard. And Obama and the spending interests failed, and Republicans and the taxpayers won,” he said.

He cited several recent examples of Republican primaries in which ATR-backed candidates defeated Republicans considered less fiscally conservative. This month, ATR helped Indiana state Treasurer Richard Mourdock defeat six-term Sen. Richard G. Lugar in a Republican primary. And this week, Tom Cotton won a Republican primary in Arkansas after ATR called voters to let them know his opponent had not signed the pledge. Cotton is seeking a seat held by retiring Democrat Rep. Mike Ross.

Norquist said the ATR pledge remains the best way to signal to wary voters that a candidate will not change his mind on taxes once in office. “The pledge isn’t what keeps them from raising taxes,” he said. “It’s what confirms to voters that they won’t raise taxes. Because they’re competing with several hundreds of years of politicians lying about this.”

But a new test looms: a colossal fight over spending and taxes at the end of the year, when the Bush-era tax cuts expire at the same time a series of deep cuts to defense and domestic programs is set to take effect.

Democrats have said they will not agree to renew some of the tax breaks or avert the defense cuts, as Republicans want, unless Republicans agree to impose higher taxes on the wealthy. Any wiggle room for Republicans on taxes could dramatically reshape that debate.

Because of the GOP’s sweeping successes in the 2010 election, the NRCC’s targeted Young Gun races include some districts where Republicans have little chance of winning in November. It is unclear how many of the new pledge refuseniks will make it to Washington.

But after months of Democratic attacks on ATR and Norquist as obstacles to a debt deal, some Republican candidates report that they are hearing from more voters who want them to reject the pledge than the opposite.

Gary DeLong, a member of the Long Beach City Council who is labeled a “contender” for a House seat by the NRCC, said he is routinely encouraged on doorsteps and at town halls and candidate coffees to avoid the pledge.

Voters “ want me to represent them and not special interests,” said DeLong, who will compete next month in California’s unusual mixed-party primary for one of two spots on the November ballot in a newly drawn district.

Two Republicans vying to challenge Democratic Rep. Dave Loebsack in Iowa have indicated they will not sign. In Indiana, former U.S. attorney Susan Brooks won the GOP nomination for a solidly Republican seat, defeating three Republicans who had signed the pledge.

“She’s committed to lowering taxes,” said Dollyne Sherman, a spokeswoman for Brooks. “She thinks that’s a key ingredient in restoring the nation’s economy. But she doesn’t need to sign a tax pledge to do that.”

Sen. Tom Coburn (R-Okla.), a fiscal conservative who has tangled with Norquist, said he believes candidates are starting to understand that the ATR pledge’s power has been exaggerated by Norquist and the media and that Norquist is wrong when he asserts that it is nearly impossible to win a Republican primary without signing the pledge.

“That’s him patting himself on the back,” Coburn said. “And I think it’s bull crap.”