WASHINGTON  Just two weeks after an election that left him struggling to find his way forward, President Obama has decided to confront Senate Republicans in a make-or-break battle over arms control that could be an early test of his mettle heading into the final two years of his term.

He is pushing for a vote on a signature issue despite long odds, daring Republicans to block an arms-control treaty at the risk of disrupting relations with Russia and the international coalition that opposes Iran’s nuclear program. If he succeeds, Mr. Obama will demonstrate strength following the midterm election debacle. If he fails, he will reinforce the perception at home and abroad that he is a weakened president.

“It’s really high stakes,” said Geoffrey Kemp, a former national security aide to President Ronald Reagan and a scholar at the Nixon Center, a research group in Washington. “I would say it’s the biggest gamble he’s taken so far, certainly on foreign policy.”

After months of quiet negotiations blew up this week, Mr. Obama on Thursday escalated ratification of the agreement, the so-called New Start treaty, into a public showdown, enlisting former Republican officials and assigning Vice President Joseph R. Biden Jr. to work on it “day and night.” An allied group, the American Values Network, kicked off a television and e-mail campaign.