With so many Republican cold warriors behind it, "the argument has changed," says David Cohen, an arms-control activist and founder of Civic Ventures. Now, "it's are you a sane, prudent responsible person? Or are you going to be on the fringe?"
The White House views the treaty as a crucial step toward Obama's goal of stopping the global spread of nuclear weapons and an example that should be set by the two countries that hold 90 percent of the world's stockpile. The treaty demonstrates a commitment to non-proliferation, Rhodes says.
If the Senate doesn't vote before the November elections and Obama's Democratic party loses control of the Senate, passage could get trickier. But most experts say the treaty likely will get through with 80 or more votes.
"The American people want to see Congress accomplish something, and START is a made-to-order agreement," says Andy Johnson, head of the national security programs at the politically moderate think tank Third Way. "If the Republicans delay the process, they open themselves up to the charge of putting politics over national security."
The Russian Parliament also is likely to vote on ratification this year.
Contributing: The Associated Press