Growing Republican support for raising taxes to help reduce the deficit has prompted a GOP identity crisis, sparking a clash within the party over whether to abandon its bedrock anti-tax doctrine.

Tensions have mounted in recent days as two of the GOP’s most fervent anti-tax stalwarts on Capitol Hill — Sen. Patrick J. Toomey (Pa.) and Rep. Jeb Hensarling (Tex.) — have lobbied party colleagues behind the scenes to forgo their old allegiances and even break campaign promises by embracing hundreds of billions of dollars in tax hikes.

The two conservative lawmakers have pushed the increases as part of their work on the bipartisan congressional “supercommittee” tasked with finding at least $1.2 trillion in deficit reductions by a Thanksgiving deadline. Their plan, which also addresses entitlement spending, would generate at least $300 billion in new tax revenue over the next decade by overhauling the tax code to lower rates but also eliminate deductions and loopholes.

Their work has been met with a furious backlash as fellow conservatives inside and outside Congress expressed amazement that two of their biggest allies appear now to be foes.

“We’ve not had a conversation like this within the party in two decades,” said Rep. Patrick T. McHenry (N.C.), who on Wednesday gathered signatures from about 70 House Republican colleagues for a letter to the supercommittee leadership, calling any tax increase “irresponsible and dangerous to the health of the United States.”

Describing Hensarling as a “mentor,” McHenry added: “It’s a very tough situation.”

The conservative group Americans for Prosperity, which has fought taxes, began a campaign targeting 40 House Republicans who have expressed an openness to new taxes. The group invited constituents in Virginia and Florida to call in to telephone town hall meetings Wednesday night and purchased radio ads in five states charging that the members “don’t get it” when it comes to tax policy.

Critics say that giving any ground on taxes would violate party doctrine that has not been challenged since President George H.W. Bush broke his “read my lips” pledge as part of a 1990 budget deal.

Although it’s not clear how many Republicans are willing to raise taxes, the numbers have been growing in the House and Senate. Activists say they fear that the presence of rock-ribbed conservatives in that camp and support in the business community for a deal of some sort could be spurring widespread defections.

More Republicans moved Wednesday to push their party toward accepting new taxes — putting the issue in historic terms.

“This is about more than money,” said Sen. Lamar Alexander (Tenn.), a member of the Senate’s GOP leadership team. “It’s about whether the president and the Congress can competently govern, about whether we can face up to the biggest problem facing our country and, working together, can we solve that problem?” He added that both parties “need to put more on the table and get a result.”

As of Wednesday, the fate of the proposal backed by GOP supercommittee members was uncertain. Democrats rejected it last week, demanding more new taxes than the Republican panel members had been willing to offer. But negotiations continued, and Hensarling told reporters late in the day that he was “willing to look at any offer” from the other party.

GOP aides cautioned that the shifting ground on taxes has come about only because of the unique nature of the supercommittee.

Party leaders wary of Congress’s dismal approval ratings are loath to appear incapable of deficit reduction. Moreover, Republicans are worried about the deep automatic spending cuts to defense programs that would be triggered if the committee fails to reach a deal.

Still, officials and leading activists conceded Wednesday that even in this unusual context, the willingness of conservatives to compromise on taxes could forever change the debate within the party.

After all, it was Toomey, who, before his 2010 campaign for Senate, headed the Club for Growth, a pro-business group and pillar of the conservative economic movement. He has been one of Washington’s leading advocates of tax cuts.

“You’ve got Toomey, the most conservative member of the supercommittee, saying that under certain circumstances he’d be willing to raise taxes. That’s a big deal,” said Phil Kerpen, vice president of Americans for Prosperity.

“The most important part of the Republican brand is that they won’t raise taxes,” Kerpen added. “Some say this is a unique situation. Well, people won’t get the nuances. They’ll just see both parties are willing to raise taxes.”

Toomey could not be reached for comment Wednesday. He told “Fox News Sunday” over the weekend that his plan, which he said was “contingent on pro-growth tax reform,” was better than striking no deal with Democrats.

Any loosening of the GOP’s firm stance on taxes would sharply alter the political dynamics of the party. This change would mark a turnaround, just a year after the rise of the staunchly anti-tax tea party movement propelled Republicans into the House majority. Party activists note that Bush’s 1990 tax deal depressed conservative voter turnout in the next election, contributing to his defeat. They warn that a similar transgression could again deflate the party base, leading to President Obama’s reelection next year and costing Republicans the House.

A GOP aide, speaking on the condition of anonymity to discuss internal party deliberations, said the openness to tax increases would not “wipe away” the “very strong fundamental view about taxes.” And yet, the aide added, “It does change the conversation within the context of deficit reduction, and it will have implications.”

Adding to the dilemma facing many Republicans is their party’s long embrace of the Taxpayer Protection Pledge, which was created 25 years ago by anti-tax activist Grover Norquist and remains a rite of passage for Republican candidates at all levels. The GOP supercommittee plan appears to violate the pledge’s requirement that members oppose “any net reduction or elimination of deductions and credits, unless matched dollar for dollar by further reducing tax rates.”

Norquist said GOP leaders have assured him that they will not support a tax increase. But some in the party say they no longer feel bound by the Norquist pledge.

“I want my constituents to know that I am not in favor of raising taxes. That’s why I signed that pledge,” said Rep. Charles F. Bass (N.H.). “And I if I have to break that pledge for some reason, it would be because I think there’s a far greater good associated with it, and I’m willing to bear the consequences of that.”

Rep. Steven C. LaTourette (Ohio) signed the pledge in 1994, long before he said he could envision today’s economic turmoil. “Circumstances change,” he said.

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Staff writer Rosalind S. Helderman contributed to this report.