WASHINGTON — The Senate plans a critical vote Tuesday on the Obama administration's nuclear arms treaty with Russia, as Russia warned lawmakers not to alter the treaty's terms and the White House stepped up lobbying.

Most Senate Republicans Monday continued their so-far unsuccessful effort to try to change the pact — and deny President Barack Obama a major foreign policy victory.

The Senate is scheduled to vote Tuesday on limiting debate on the treaty, which will require support of 60 of the 100 senators. If that succeeds, a vote to ratify the treaty, which needs two-thirds of the senators, could occur later Tuesday or Wednesday, and supporters think they'll have the 67 votes they need.

Supporters ramped up pressure on Monday, as Russia warned that it wouldn't renegotiate the terms, and Adm. Michael Mullen, the chairman of the Joint Chiefs of Staff, wrote the Senate a three-page letter explaining in detail why the treaty is vital to U.S. national security.

"I can only underscore that the strategic nuclear arms treaty, worked out on the strict basis of parity, in our view fully answers to the national interests of Russia and the United States," Russian Foreign Minister Sergei Lavrov said in an interview with the Interfax news agency.

At the White House, Obama, too, kept pushing, making phone calls to individual senators, as did Vice President Joe Biden.

"If there are issues in which people have questions," said White House Press Secretary Robert Gibbs, "we are certainly happy to provide answers either through the president or the president could have other people call them." Gibbs said the White House is confident that the Senate will ratify the treaty before it adjourns.

At the Capitol, Republican opponents kept up their protests.

"Our top concern should be the safety and security of our nation, not some politician's desire to declare a political victory and host a press conference before the first of the year," said Senate Republican leader Mitch McConnell of Kentucky.

Democrats, as well as a handful of Republicans, voiced dismay.

"I scratch my head," said Senate Foreign Relations Committee Chairman John Kerry, D-Mass., because "the national security interests of our country are going to get wrapped up in ideology and politics."

To some outside analysts, the GOP opposition didn't seem to be about principle, not least because five secretaries of state for past Republican presidents all have endorsed the treaty.

The Republican effort to derail the treaty is "99 percent politics," said Steven Greene, an associate professor of political science at North Carolina State University. "You have former secretaries of state and people from the national security apparatus in Republican administrations behind this. I haven't seen anything to suggest this is anything but politics."

Neither has former Sen. Chuck Hagel, a Nebraska Republican and a former member of the Senate Foreign Relations Committee. He called some GOP objections to the treaty "exaggerated rhetoric" and an attempt "to take away a so-called victory from the president" on an important foreign policy issue.

While some senators may have legitimate reservations about the treaty, Hagel said it's time for "the leadership on the Republican side to rise above politics" and move to a final vote.

"The number of hearings this treaty has been subjected to, if not a record, is close to one," Hagel said. "I find it ridiculous — 'We need more time, we need more time.'"

The New START Treaty would restrict the U.S. and Russia at the end of a seven-year period to deploying no more than 1,550 strategic nuclear warheads on 700 strategic bombers, intercontinental ballistic missiles and submarine-launched missiles.

That's about 30 percent below the 2,200 warhead cap set in a 2002 treaty that's due to expire at the end of 2012. The new treaty also would allow the two sides to resume inspections of each other's nuclear weapons, which have been suspended for more than a year, a gap that worries U.S. intelligence officials.

The new treaty also includes an inspection system that would be more intrusive than the regime that ended last year. Among other measures, U.S. and Russian experts would be allowed for the first time to look inside the other's missiles and count the actual number of warheads they carry, rather than accept agreed-upon assumptions as they did before.

It's rare for senators to reject such treaties. The last was the Comprehensive Nuclear Test Ban Treaty in 1999 during Bill Clinton's presidency. It failed on a 48-51 vote.

Monday, the Senate considered amendments to the treaty that supporters said would in effect kill it. None succeeded.

In Washington, some experts questioned McConnell's assertion that the treaty "does nothing to significantly reduce the Russian Federation's stockpile of strategic arms."

"It's ridiculous. It sounds like he is a little deficient in mathematics here," said Robert Norris of the pro-arms control National Resources Defense Council. "That's the whole purpose of the treaty, to reduce the number of warheads."

Meanwhile, in another part of his Interfax interview, Russia's Lavrov rejected the Obama administration's assertion that the Senate's failure to approve the treaty would hurt its efforts to "reset" U.S. relations with Russia.

"Nuclear disarmament is only one of the key areas, but far from the only area of our cooperation with the United States," he said.

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