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…

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…

“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.”