Anti-tax proponent Grover Norquist is warning Republican lawmakers returning to Congress from the Thanksgiving recess that reneging on the Taxpayer Protection Pledge that he convinced most of them to sign could prompt voters to boot them from office.

A small, but increasing number of Senate Republicans have declared they are willing to raise taxes if it will result in a deal that would avert the so-called fiscal cliff of massive tax hikes and automatic spending cuts that economists say would do significant harm to the struggling economy.

The lawmakers said they may support increasing tax revenue through closing loopholes in the tax code.

The most recent defection came on Monday when Sen. Bob Corker, R-Tenn., told CBS News he is not obligated to uphold the pledge written by Norquist in 1986 and signed by nearly every GOP lawmaker.

But Norquist, whose Americans for Tax Reform pledge allows for "no exceptions" for raising taxes, threatened dire consequences for the dissenters, particularly those lawmakers who want to get re-elected.

"Corker was elected to the Senate because he took the pledge," Norquist told Fox News on Monday. "If he breaks it, he is going to have to have a conversation with the people of Tennessee about keeping his word."

Sens. Saxby Chambliss, of Georgia, John McCain, of Arizona, Lindsey Graham, of South Carolina, Lamar Alexander, of Tennessee, and Mike Crapo, of Idaho, have also signaled their willingness to raise revenue via a tax increase. In the House, Rep. Peter King, R-N.Y., dismissed the Norquist pledge, saying that "the world has changed, and the economic situation is different."

Republicans softened their stance on taxes since Democrats held onto the Senate and White House in the election earlier this month. The party is eager to avoid being blamed for tax rate increases and cuts that would take effect in January if the two parties can't strike a deal.

House Speaker John Boehner, R-Ohio, announced a day after the election that he would support a compromise deal to avert the fiscal cliff that raises revenue through closing tax loopholes but which also makes significant cuts to the federal budget and reforms entitlement spending to make it more cost-effective.

Republican strategist Ron Bonjean, who served as a top leadership aide in both the House and the Senate, pointed out that Republicans are not calling for tax rate increases and are not departing from where the GOP leadership stood in 2011, when during the talks to raise the nation's debt ceiling Boehner proposed a similar plan to close loopholes in order to raise revenue.

"The key is in the fine print of any potential fiscal cliff deal, because these Republicans aren't saying anything different than Speaker Boehner did when he offered to compromised on tax revenue as long as there is entitlement reform," Bonjean said.

In an interview with The Washington Examiner, Chambliss said he would not back a tax rate increase. Corker told The Examiner he was also opposed to a tax rate increase but believed all options should "be on the table" during the negotiations.

Democratic Majority Leader Harry Reid, of Nevada, and White House spokesman Jay Carney both declared Monday that no bill would be signed into law unless it raises tax rates on upper-income earners.

Republican Minority Leader Mitch McConnell, R-Ky., meanwhile, reiterated the loophole closure offer, saying Republicans have been willing to move outside their "comfort zone" but have not compromised their principals in an effort to avert the fiscal cliff.