The Senate Parliamentarian has determined that the new Strategic Arms Reduction Treaty can be amended on the floor, potentially throwing another wrench into ratification during the lame-duck session.

Specifically, the parliamentarian ruled that START’s controversial preamble document can be changed by Senators during the floor debate. This could prove significant because President Barack Obama and Senate Republicans have been at odds over key portions of the preamble, and Tuesday’s ruling could provide the GOP with more leverage to push ratification to next year.

“This is big news,” a senior Republican Senate aide said, who noted that the parliamentarian’s ruling came after an inquiry was made.

Ratification of the treaty requires 67 votes. One area of concern for Republicans has been the language of the treaty’s preamble addressing U.S. missile defense capabilities. The Obama administration has defended this portion of the preamble, and a concerted effort by Republicans to amend it could create a further delay, particularly with the clock winding down on the lame duck.

Obama signed START with Russian President Dmitry Medvedev earlier this year, and has made ratification of the treaty in 2010 his top foreign policy priority.

Earlier Tuesday, Jim Manley, the chief spokesman for Senate Majority Leader Harry Reid (D-Nev.), said START could be introduced on the floor as soon as Wednesday. But Senate Minority Whip Jon Kyl (Ariz.), the lead Republican negotiator on the treaty, has made clear that there will not be enough GOP votes to ratify it if Democrats try to rush the debate.

The Senate is scheduled to adjourn for the year Friday, although that could change. The Senate still has to vote on final passage of the tax cut extension bill, as well as a controversial and potentially time-consuming government-spending bill. Senate Foreign Relations Chairman John Kerry said Tuesday that ratification of START is still possible this year.

“We want to get it done,” the Massachusetts Democrat told reporters before the weekly Democratic caucus lunch. “But the key is, when we wrap up what.”