Senate Democrats will move ahead on a U.S.-Russia nuclear treaty, President Barack Obama's top foreign policy priority, despite strong opposition from some Republican lawmakers, AP quoted a congressional aide as saying Tuesday. The White House has signaled that Obama would delay his holiday vacation to ensure ratification of the treaty that would limit both nations' nuclear warheads and establish a system for verification. Congress is struggling to complete several top pieces of legislation, including a tax cut bill and a measure to keep the government running, in the final week of a year-ending congressional session. The treaty is one of the items on the Senate Democrats' must-do list. Jim Manley, a spokesman for Senate Majority Leader Harry Reid, a Democrat, said Tuesday that the Senate would begin debate on the treaty as early as Wednesday. Obama has pushed hard for ratification of the treaty, which has the backing of former Presidents George H.W. Bush and Bill Clinton as well as secretaries of state and defense for Republican and Democratic administrations. But several Republican senators have expressed concern that the treaty would limit a missile defense system and have suggested there is not enough time in the current session to give the pact the attention it requires. Twenty-two Republican senators signed a letter Dec. 2 calling for consideration of the treaty to be delayed until next year. Obama has gained support for the pact in recent days and is within striking distance of the 67-vote threshold the U.S. Constitution requires for the Senate to ratify a treaty. Maine's two Republican senators, Olympia Snowe and Susan Collins, indicated their support last week, and at least six other Republicans have signaled backing for the treaty, although with some qualifications. All 58 senators in the Democratic caucus are expected to vote for the treaty. «I believe we can pass the START treaty if we get a chance to do it,» Foreign Relations Committee Chairman John Kerry, also a Democrat, said Tuesday. The administration is pressing to complete the treaty this year because the political calculation becomes more difficult in the next Congress, when Republicans increase their numbers in the Senate. Obama signed the treaty with Russian President Dmitry Medvedev in April. The treaty would allow each country 1,550 strategic warheads, down from the current ceiling of 2,200.