WASHINGTON — The Senate’s Republican leader said Sunday he would oppose a nuclear arms treaty with Russia, complicating President Obama’s drive to secure a foreign policy victory in the final days of the postelection Congress. Senior Democrats still expressed confidence the Senate would ratify the accord and pushed for a showdown vote early this week.

The White House and Democrats are determined to win approval of the landmark treaty before January, when Republicans increase their numbers in the Senate, dimming its outlook. During a rare Sunday debate, Democrats beat back a GOP amendment to change the treaty, which would have effectively killed it. Senate Majority Leader Harry Reid, D-Nev., set a vote for Tuesday to end the debate and move to a final vote.

“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 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.”

Hours earlier, Minority Leader Mitch McConnell, R-Ky., dealt a blow to the administration’s hopes for strong bipartisan support, criticizing the treaty’s verification system and expressing concern that the pact would limit U.S. missile defense options even though Obama insisted Saturday that the treaty imposes no restrictions on missile defense.

“Rushing it right before Christmas strikes me as trying to jam us,” McConnell said on CNN’s “State of the Union.”

“I think that was not the best way to get the support of people like me.”

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 agreement.

Democrats expect to get 57 votes from their caucus, with Sen. Ron Wyden, D-Ore., absent next week due to cancer surgery. Four Republican senators — Richard Lugar of Indiana, Olympia Snowe and Susan Collins of Maine and George Voinovich of Ohio — have said they back the treaty.

While backers fretted over McConnell’s decision, several Republicans said Obama’s letter to congressional leaders Saturday vowing to move ahead on missile defense carried considerable sway.

“It takes care of me,” said Sen. Robert Bennett, R-Utah, who indicated he was leaning toward voting for the treaty. Snowe said it was “important for the president to be emphatic with respect to missile defense and modernization” of the remaining nuclear arsenal. Voinovich welcomed the statement.

Sen. John McCain of Arizona, said he was still undecided.