WASHINGTON — President Barack Obama today sternly proclaimed ratification of the U.S.-Russia nuclear arms treaty “a national security imperative,” saying the Senate must act before Congress goes home for the year.

Lobbying hard for votes from the White House, Obama expressed confidence he would be able to get enough votes to get the treaty passed, saying that every president since Ronald Reagan has been able to get such pacts through.

To apply pressure on the Senate, Obama surrounded himself for his statement with former secretaries of states and defense of both parties, who all support the treaty, along with leaders of his administration.

He said the United States “cannot afford to gamble” with its ability to verify Russia’s strategic nuclear arsenal — a central element of the treaty. Nor, he said, can this country risk losing Moscow’s support on other matters of national security, including pressure on Iran over it’s pursuit of nuclear weapons.

“This is not about politics,” Obama said. “It’s about national security. This is not a matter that can be delayed.”

The president insisted that the administration has taken the necessary time to do the treaty the right way, even as a key Republican senator on the matter, Jon Kyl of Arizona, has said that ratification should not be rushed during the current lame duck session.