WASHINGTON -- Democrats have moved a step closer toward a crucial U.S. Senate vote on a new arms control treaty with Russia, beating back Republican efforts to alter the accord and setting up a showdown with the Republican Party on President Barack Obama's top foreign policy priority.

The White House has made ratification of the landmark agreement an imperative in the closing days of the postelection Congress, but its hopes for the pact were complicated Sunday as Senate Minority Leader Mitch McConnell said he would oppose the treaty.

"Rushing it right before Christmas strikes me as trying to jam us," said McConnell. "I think that was not the best way to get the support of people like me."

McConnell on CNN's "State of the Union" criticized the treaty's verification system and expressed concern that the pact would limit U.S. missile defense options even though Obama insisted Saturday that the treaty imposes no restrictions on the system aimed at protecting the United States and its allies from ballistic missile attacks.

Undeterred by McConnell's opposition, Majority Leader Harry Reid announced Sunday night that the Senate would vote Tuesday to end debate on the treaty and move to a final vote.

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"It is time to move forward on a treaty that will help reverse nuclear proliferation and make it harder for terrorists to get their hands on a nuclear weapon," Reid said, adding that the debate soon "will come down to a simple choice: you either want to keep nuclear weapons out of the hands of terrorists, or you don't."

A sixth day of debate was scheduled for Monday that included a closed session to discuss intelligence issues.

The White House and Democrats are determined to win approval of the treaty before January, when Republicans increase their numbers in the Senate, dimming its outlook. During a rare Sunday session of the Senate, Democrats turned back a Republican amendment to change the treaty, which would have effectively killed it.

Just weeks after Obama's self-described "shellacking" in the Nov. 2 midterm elections, ratification of the treaty would cap a string of political victories for the White House. Congress endorsed the president's tax compromise with Republicans and voted Saturday to repeal the military's ban on openly gay servicemembers.

While McConnell's opposition did not come as a surprise, it unnerved the treaty's backers, who wondered how hard he would work to defeat the accord. Treaties require a two-thirds majority of those voting in the Senate, and Republican votes are critical to Obama's success in getting the landmark agreement.

Obama and Russian President Dmitry Medvedev signed the accord -- it is known as New START -- in April. It would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200. It would also establish a system for monitoring and verification. U.S. weapons inspections ended a year ago with the expiration of a 1991 treaty.

Proponents of the treaty, including much of the military and foreign policy establishment, cite the renewed weapons inspections and say the pact would keep the two biggest nuclear powers on the path to reducing their arsenals.

After several hours of debate Sunday, the Senate voted 60-32 to reject a measure to add language on tactical nuclear weapons to the treaty's preamble, which would have forced it back to negotiations, dooming the accord.