Languishing at the bottom in polls of the Republican presidential field, Gov. Chris Christie of New Jersey ramped up his tough talk on foreign policy on Monday, calling President Obama a “weakling” and saying that the United States should threaten to shoot down Russian planes conducting airstrikes in Syria.

“My first phone call would be to Vladimir, and I’d say, ‘Listen, we’re enforcing this no-fly zone,'” Mr. Christie said on MSNBC. “And I mean we’re enforcing it against anyone, including you. So don’t try me. Don’t try me. Because I’ll do it.”

Mr. Christie went on to say that Mr. Putin was “punching above his weight” and called Mr. Obama “this weakling in the White House.”

In a separate interview on Fox News, Mr. Christie said the United States faced a choice between letting Russia try to reconstitute the Soviet Union by expanding its influence in the Middle East or engaging in a military confrontation. Asked whether he thought it was wise to engage in a military conflict with Russia if it breached such an American no-fly zone, Mr. Christie said it would be necessary.

“You take him down,” Mr. Christie said, referring to Russian warplanes.

While Mr. Christie has been critical of his rivals for being overly cautious when it comes to the crisis in Syria, he faces the prospect of missing the main stage at the Republican debate this month because his poll numbers are so low. However, on Monday the governor expressed confidence that he was poised for a comeback.

“I don’t mind the polls,” Mr. Christie said, acknowledging that he would be nervous if voting were held tomorrow. “There’s no one on that stage who’s been more tested than me.”