NEW START RATIFICATION COMES INTO FOCUS…. It was a busy day for the pending nuclear arms treaty, New START, in the Senate yesterday. President Obama, Vice President Biden, and Secretary of State Hillary Clinton furiously worked the phones, urging Republican senators to do the right thing; the Senate met in closed session so members could hear classified information related to the treaty; and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, wrote a letter to each senator, imploring them to ratify the measure, calling it “vital” to U.S. national security.

And by late yesterday, the finish line was finally in sight.

The Senate moved closer on Monday to approving a new arms control treaty with Russia over the opposition of Republican leaders as lawmakers worked on a side deal to assure skeptics that the arms pact would not inhibit American plans to build missile defense systems.

A Republican senator announced that he would vote for the treaty and two others said they were leaning toward it after a closed-door session on classified aspects of the pact. At the same time, Senator John McCain, Republican of Arizona, produced separate legislation that could reassure fellow Republicans worried about the treaty’s impact on missile defense.

By the end of another tumultuous day, treaty backers said they could count more than the two-thirds majority required for approval in votes that could begin as early as Tuesday.

The goal, of course, remains 67 votes. The 58 members of the Senate Democratic caucus are already on board, meaning the treaty will need a minimum of nine Republican votes. As of this morning, there appear to five firm “aye” votes from the GOP caucus: Lugar, Snowe, Collins, Brown, and Voinovich. As the day progressed yesterday, Republicans Gregg, Bennett, Isakson, and Corker also said they’re likely to support the treaty, while Murkowski and Cochran also appear to be leaning in the right direction.

If they all follow through, that’s 11 Republican votes, which would be more than enough to succeed, and likely cause a few more GOP members to come along, rather than go on the record defying the pleas of the Pentagon (again).

And what about this reported side deal to make the McCain contingent happy? Apparently, they want some kind of formal assurances that the U.S. would pursue missile defense in Europe, regardless of Russian objections, which would not require formal changes to New START itself. The proposal is being pushed by McCain, Graham, Kyl, and Kirk, though it’s unclear if any of the four would end up supporting ratification, even if their plan were adopted, and the White House has not yet commented on their offer.

Regardless, at this point, the votes appear to be in place whether this additional amendment is approved or not.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.