WASHINGTON — The Obama administration pressed senators Thursday to support a nuclear pact with Russia that has been a top foreign policy priority, while some Republicans attempted to kill the deal with amendments.

The administration sent its chief negotiator on the treaty and other officials to Capitol Hill to respond rapidly to criticism of the treaty and attempt to win over more Republicans. All Democrats and independents were expected to support the New START deal, but it needs Republican votes to pass.

Defense Secretary Robert Gates and Marine Gen. James Cartwright rejected Republican charges that the agreement would curtail American missile defense options. "This treaty in no way limits anything we want or have in mind on missile defense," Gates said Thursday.

Senate Democrats brought the treaty up for debate Wednesday reflecting confidence that they could deliver approval after the pact appeared stalled for weeks.

Ratification of the agreement along with a tax deal with Republicans nearing approval would represent major victories for Obama on both his top foreign and domestic legislative priorities just weeks after his Democratic party suffered steep losses in congressional elections.

The White House and senior Democrats believe they have the two-thirds majority, or 67 votes, needed for ratification in the Senate. In a fresh sign of momentum, Democrats easily prevailed, 66-32, to move forward on the pact, winning the support of nine Republicans. Among them was Sen. John McCain, Obama's 2008 presidential rival and a top lawmaker on national security issues.

On Thursday, senior administration officials were pressing GOP lawmakers to shore up votes.

Several Republicans, led by Sen. Jon Kyl of Arizona, objected to considering the treaty in the waning days of Congress' short year-ending session, insisting the Senate should wait until next year. Sens. Lamar Alexander and Saxby Chambliss said they could support the treaty but not under the current timetable. Alexander told reporters it was "reckless."

In a positive sign for the treaty's prospects, Republicans backed down on a threat Thursday to have it read in its entirety on the Senate floor, which could have caused significant delay.

Obama has said he is prepared to put off his holiday vacation travel until the treaty is completed, elevating the measure to year-end, must-do status along with the tax deal he cut with Republicans. Democrats are determined to push the treaty through the Senate to give Obama a foreign policy victory before the Republicans take more power next year.

Some Republicans were preparing amendments to the body of the treaty, which would serve to kill it because any changes would require new negotiations with Russia. Still, those efforts were likely to be defeated.

Democrats were offering time to debate amendments to a separate document that would be approved along with the nuclear deal and serve in part as a congressional commentary on the treaty.

Republican opponents aired criticisms of the treaty Thursday on the Senate floor. Kyl charged amid a long list of objections that negotiators had won few concessions from Russia.

"They get everything out of it," he said. "I don't know what we get out of it except for the president to say he made another arms control deal with Russia."

The administration responded rapidly. "We don't do these treaties as a favor to the Russians," chiefs negotiator Rose Gottemoeller said. "They are in our national security interests."

Proponents of the treaty cite the benefits of weapons inspections and say that it would keep the two biggest nuclear powers on the path to reducing their arsenals. Opponents have asserted it would limit U.S. missile defense options and that it has insufficient procedures to verify Russia's adherence.

The treaty has pitted moderate Republicans against hard-line conservatives, with potential 2012 challengers to Obama making opposition a requirement for anyone weighing a bid for the Republican presidential nomination. Lining up in opposition are Sarah Palin, Mitt Romney, Newt Gingrich, Minnesota Gov. Tim Pawlenty and Sen. John Thune of South Dakota, all mentioned as possible Republican candidates to take on Obama as he runs for a second term.

Backing the treaty are former Presidents George H.W. Bush and Bill Clinton and several secretaries of state and defense from Republican and Democratic administrations, including former Secretary of State Condoleezza Rice.

Supporters are pushing for ratification in this legislative session because prospects for passage will dim when the Democrats' majority shrinks by five senators in January.

Obama signed the treaty with Russian President Dmitry Medvedev in April. It would allow each country 1,550 strategic warheads, down from the current ceiling of 2,200.

Associated Press writer Julie Pace contributed to this report.