WASHINGTON - The fate of a US-Russia nuclear arms control treaty, President Barack Obama's top foreign policy priority, may now hinge on the fate of his top domestic goal, winning approval of a contentious tax cut plan.

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If Obama persuades reluctant congressional Democrats to quickly endorse the tax cuts he negotiated with Republicans, that would clear the way for the Senate to consider the New START treaty.

But if the Senate gets bogged down in a tax debate, there will not be time to consider the treaty before the year ends and Congress goes out of session. With fewer Democrats in the Senate next year, prospects for passage dim.

A failure to ratify the treaty could cast a long shadow over Obama's foreign policy goals. Obama has made arms control one of his top priorities and the centerpiece of his efforts to improve relations with Russia.

Treaty supporters said the tax deal Obama reached with Republicans increased the New START's prospects.

"I thought this was good news, that they are closer to a vote," said Kay King, a specialist on Congress and foreign policy at the Council on Foreign Relations. "But it's not done yet."

Still, it is not clear how quickly the tax-cut issue would be resolved. Some Democrats, angered because they believe Obama made too many concessions on tax cuts for the rich, were calling for changes.

The debate comes as treaty supporters believe they have had some momentum in cornering enough votes for ratification. Obama would need at least nine Republicans to join all Democrats and independents to reach the necessary two-thirds majority, or 67 votes, in the Senate. That number would grow to 14 when the new Congress begins in January.

On Tuesday, Republican Judd Gregg said in a television appearance that he was leaning toward supporting it, joining a growing list of Republicans who have spoken favorably on the treaty.

Republican Sen. George Voinovich, who had recently raised concerns about the treaty, also said he would like to see the treaty ratified this year.

"It's a modest proposal, we ought to get it done," he said.

Former Secretary of State Condoleezza Rice also joined a long list of Republican foreign policy luminaries backing the treaty Tuesday. In a Wall Street Journal opinion piece, she said lawmakers should ratify the treaty while making a clear statement that it will not limit US missile defense options.

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.

Democrats were increasingly confident that the treaty could be ratified, if it could be brought for a vote. But once the tax cut issue is resolved, the treaty could also face stalling tactics from Republican opponents, who have argued that it would undermine US security interests.

Lawmakers also need to pass a measure that would finance government operations before the end of the year. Republicans have insisted that it should be dealt with before any vote on the treaty.

Some Republicans, led by Sen. Jon Kyl, have said that the treaty could be considered next year. At a minimum that would delay ratification by months, because new hearings would have to be held before it could be brought to the floor.

On Tuesday, Kyl said he still does not believe there is time to consider the treaty this year.