A prominent conservative political action committee is vowing to defeat any Republican senator who votes to ratify the New START nuclear arms control treaty with Russia in the lame-duck session of Congress.

The National Republican Trust PAC is making its pledge in a letter that was obtained by RealClearPolitics before it was set to be delivered on Friday afternoon.

In the letter, the PAC's executive director Scott Wheeler writes to National Republican Senatorial Committee Chairman Sen. John Cornyn and Republican National Committee Chairman Michael Steele that the group is "fully committed" to defeating any GOP senator that votes for ratification during the lame-duck session.

"We as a country cannot allow our national defense to be greatly undermined by a President and members of a lame duck Congress who are hell-bent on redistributing America's military advantage because of their bizarre belief that the US lacks the moral authority to be the world's lone superpower," the letter says. "Republicans cannot and must not participate in the sell out of our national defense merely because of political pressures applied to them by an extremist President and a liberal media."

In the letter, Wheeler goes on to write that the group will recruit primary challengers and "will be singular in the purpose" of defeating pro-ratification Republican senators.

"Senator Cornyn and Mr. Steele -- we urge you to advise every Republican Senator to prevent this treaty from coming to the Senate floor for a vote," the letter says. "Now is time to put our future before politics. Now is the time to do the right thing and stand firm in our beliefs."

The New START treaty, which President Obama and Russian President Dmitry Medvedev signed in April, would limit both countries' nuclear arsenals to 1,550 warheads, put new caps on missile launchers and heavy bombers, and would establish verification regimes for compliance.

The White House has made ratification of the treaty during the lame-duck session a top priority and has cited extensive support within the Republican foreign policy establishment, including the backing of every living GOP secretary of state and former President George H.W. Bush.

Sen. Majority Leader Harry Reid on Friday vowed to bring the treaty to a vote next week before Congress adjourns.

The National Republican Trust PAC objects to the treaty on several grounds, including its concerns that it could limit U.S. missile defense capabilities.

"Clearly scrapping our missile defense system, allowing Russia to dictate what we can do in that regard, is absolutely foolish," Wheeler said. "Even if Russia did abide by this treaty -- and they have a long history of not abiding by them -- even if they did, it leaves us vulnerable to a number of other threats, including China and Iran."

New START must receive at least 67 votes in the Senate before being taken up by the Russian Duma, where it would be expected to face relatively easy passage. Its prospects for Senate ratification remain uncertain.

Wheeler said that his group was earnest in its vow to mount credible primary challenges, pointing to radio ads and robocalls it launched in Pennsylvania last year against Sen. Arlen Specter (D-Penn.) after he voted in favor of the stimulus bill. Facing a Republican primary challenge from Senator-elect Pat Toomey, Specter left the GOP and went on to lose a Democratic primary battle.

Wheeler pointed to Republican Sens. Olympia Snowe (R-Maine) and Bob Corker (R-Tenn.), both of whom are up for re-election in 2012, as particular targets of his group's open letter.

"I think they should look at what happened to Arlen Specter," he said. "That was us, and this is even more important than the stimulus."