"The key point here is this is not about politics -- it's about national security," Obama said. "This is not a matter that can be delayed."
Obama this weekend is scheduled to meet with Russian President Dmitry Medvedev at a NATO conference in Portugal.
The two leaders previously struck a deal aimed at reducing their respective nuclear weapons stockpiles by about 30 percent, and instituting new procedures for mutual inspection and verification.
Last week, at the Asian-Pacific Economic Cooperation summit in South Korea, Obama assured Medvedev that Senate ratification of the treaty was a "top priority."
The Strategic Arms Reduction Treaty is the cornerstone of Obama's nuclear disarmament program, a legacy-building initiative that until now drew only muted response from Republicans.
But with the Medvedev meeting looming and White House pressure building for Senate action during the current lame-duck session, more Republicans are balking. Chief among them is Sen. Jon Kyl of Arizona, a leading voice on nuclear weapons in the Senate, who said he does not believe there is enough time this year to address his concerns about the pact.
That would push the vote off until January when Republicans will have an increased presence in the Senate and chances of mustering the 67 votes needed to ratify the treaty would be even slimmer.
To make his case, Obama gathered former Secretaries of State Madeleine Albright, James Baker III and Henry Kissinger along with former Defense Secretaries William Cohen and William Perry and former National Security Adviser Brent Scowcroft for a largely symbolic meeting in the Roosevelt Room.
In brief remarks, Obama twice invoked former President Reagan's iconic admonition "trust but verify" when negotiating weapon treaties with Russia.
"In order for us to verify, we've got to have a treaty," Obama said.
While stopping short of publicly criticizing Kyl and other Republicans, White House press secretary Robert Gibbs called Senate ratification a "no-brainer."
Still, it's clear the White House is increasingly frustrated. Michael O'Hanlon, a defense policy expert at the Brookings Institution, called Kyl "a serious guy" and said the differences are not pure politics.
"You have to give the GOP credit because they have been bipartisan on foreign policy," O'Hanlon said. "Through the first two years on Afghanistan, Iraq, defense, keeping [Defense Secretary Robert] Gates -- this has not been where we had the big partisan fights."
But Cal Jillson, a political scientist at Southern Methodist University, said the current delay of START is part of a larger Republican strategy to deny Obama any meaningful victories.
The old Washington political chestnut that "politics ends at the shoreline," meaning that foreign policy is largely exempt from partisan debate, hasn't been operative for years, Jillson said.
"We used to think that meant the Atlantic, but it could be the Mississippi," Jillson said. "It could be the Snake River in Idaho." email@example.com
the Atlantic, but it could be the Mississippi," Jillson said. "It could be the Snake River in Idaho."