President Barack Obama's call for a world without nuclear weapons divided Republicans and Democrats on Friday as the Senate slogged through debate on a new arms control treaty with Russia.

Republican Sen. Jon Kyl of Arizona argued that the treaty would be a step toward an unrealistic quest.

"I think it's difficult if not impossible to achieve and I question whether it's a good idea at all," Kyl said, arguing that the goal and the treaty divert attention from dealing with real national security threats such as Iran and North Korea.

Obama has acknowledged that eliminating nuclear weapons is unlikely in his lifetime. The treaty to cap nuclear warheads for both countries and allow weapons inspections would be a very modest step toward that goal.

Democratic Sen. John Kerry of Massachusetts dismissed the criticism as irrelevant and argued that many presidents, Republicans and Democrats, have talked about the possibility of a world without nuclear weapons.

"For heaven's sake, it's incredible to me that you can't imagine and have a vision of the possibility of a world in which you ultimately work to get there. That's the purpose of human endeavor in this field," Kerry said.

In Prague on April 5, 2009, Obama said it was "America's commitment to seek the peace and security of a world without nuclear weapons. I'm not naive. This goal will not be reached quickly — perhaps not in my lifetime."

Obama has made the treaty a must-do item in Congress' lame-duck session and Senate Democrats have said they are ready to vote. But Republicans have pressed to delay action until January, when prospects for the treaty will dim as Republicans increase their numbers.

After two days of debate, Republicans offered an amendment, a measure sponsored by Sens. John McCain of Arizona and John Barrasso of Wyoming, that would effectively kill the treaty. The amendment would alter the pact's preamble to exclude a statement on missile defense. Changes would send the treaty back to negotiations with Russia, which would unlikely agree.

Republicans argue that the treaty would limit U.S. missile defense options, a notion that Defense Secretary Robert Gates and other top military leaders rejected on Thursday.

Time is an issue, as the current Congress grapples with a number of pressing items that must be addressed before the end of the year. Proponents of the treaty are insisting this Congress vote on it before the Democrats' majority shrinks in early January.

The Constitution requires approval by two-thirds of the Senate to ratify a treaty.

The treaty, signed by Obama and Russian President Dmitry Medvedev in April, would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200, and establish a system for monitoring and verification. U.S. weapons inspections ended a year ago with the expiration of the 1991 arms control treaty.

Supporters are pushing for ratification in the closing days of the year because prospects for passage will dim when Republicans increase their numbers by five senators in January.

Backers of the pact and the Obama administration were encouraged by a 66-32 vote on Wednesday to move ahead on debate, boosting Senate Democratic leader Harry Reid's contention that he has the votes for ratification.