WASHINGTON -- President Obama gained ground in his push for Senate ratification of a stalled nuclear treaty as once-reluctant Republicans signaled a willingness to back the pact with Russia.

The No. 3 Republican in the Senate, Lamar Alexander, said Thursday he is "wide open" to supporting the treaty if the administration addresses his concerns about modernization of the remaining U.S. nuclear arsenal. He praised the White House for working with lawmakers.

"They're making important steps in the right direction," Alexander said on MSNBC. He said the treaty "has important advantages to our country in terms of the data and the verification."

The treaty would cut the limits on strategic warheads to 1,550 for the United States and Russia from the current ceiling of 2,200. The pact also would establish new procedures to allow both countries to inspect each other's nuclear arsenals to verify compliance.

The administration jump-started the treaty with a series of steps this week, including outreach by Vice President Joe Biden to lawmakers and the circulation of a letter from the heads of the three U.S. nuclear weapons laboratories expressing support for Obama's 10-year, $84 billion plan to maintain the nuclear stockpile.

"Do I feel any movement on START, the answer is yes," Democratic Senate Majority Leader Harry Reid told reporters.

The laboratory directors from Lawrence Livermore, Sandia and Los Alamos said the administration's plan "would enable the laboratories to execute our requirements for ensuring a safe, secure, reliable and effective stockpile."

Sen. Richard Lugar, the top Republican on the Foreign Relations Committee and a proponent of the treaty, distributed the letter to more than a dozen Republican lawmakers at a closed meeting late Wednesday. Several Republican senators, including Olympia Snowe, emerged from the session more positive about completing the treaty in the lame-duck session.

"Speaking for myself, I think there is that reflection and recognition that we can get it done this year," Snowe said.

Republicans, led by Sen. Jon Kyl, have rejected Obama's insistence that the treaty must be dealt with before the new Congress starts next year. Some have raised concerns that the treaty would limit work on a missile defense system, and they have pressed for sufficient funds for modernization of the existing nuclear stockpile.

Kyl told Fox News' Greta Van Susteren in an interview aired Thursday night that he thought Republicans would be happy to give the White House an agreement to consider the treaty around March, as long as the new senators coming in January were adequately briefed.

Sen. Bob Corker, a member of the Foreign Relations Committee, said, "It's moving in a very positive way, but there are still some issues to be resolved." He mentioned Republican concerns about missile defense.

Citing that subject, Jim DeMint threatened to use stalling tactics to hold up ratification. John Thune, a potential Republican 2012 presidential candidate, also reiterated his opposition to moving ahead on the treaty.

Foreign Relations Committee Chairman John Kerry said in a speech on the Senate floor Thursday that he was encouraged by the discussions involving Republican lawmakers and the administration. He said he was hopeful for a "positive outcome, and we're certainly going to work in good faith to try to make that happen in the next days, hours."

Backers of the treaty circulated an op-ed from The Washington Post in which five Republican former secretaries of state urged the Senate to ratify.

"We have here an agreement that is clearly in our national interest, and we should consider the ramifications of not ratifying it," wrote Henry Kissinger, George Shultz, James Baker, Lawrence Eagleburger and Colin Powell.

Countering that argument, former Reagan administration officials Edwin Meese and Richard Perle wrote an op-ed in The Wall Street Journal saying the pact falls short of those negotiated by President Ronald Reagan and they doubt he would have supported it.