Now, that Russia-focused policy is back for those hoping to reassemble Mr. Obama’s coalition. In sharp contrast to Mr. Trump, who has never been openly critical of Mr. Putin, several said they would expose the Russian leader’s links to corruption and freeze his bank accounts. (That was a measure briefly considered — and rejected — by Mr. Obama in late 2016, as he tried to figure out how to punish Mr. Putin as the interference in the presidential election that year became clear.)

The candidates tried to outdo one another in explaining how tough they would be on Moscow.

“We have not talked about what we need to do to protect ourselves from Russia invading our election, ” said Senator Amy Klobuchar of Minnesota.

She called for the entire nation to use backup paper ballots, a step that Tom Steyer, the billionaire hedge-fund manager who gained a name — and a place on the debate stage on Tuesday night — with an expensive campaign to press for Mr. Trump’s impeachment, said he “absolutely” would freeze Mr. Putin’s assets.

But it was Syria that gave Mr. Steyer, and most of the other candidates, the opportunity they sought to present Mr. Trump as an erratic, unthinking commander in chief, incapable at looking around corners or considering the second-order effects of his actions. “Mr. Trump’s having no plans, having no process and having no partners has proved to be a disaster in Syria,” he argued. He called the president’s “America First” approach, a slogan he adopted during the 2016 campaign, “ the worst idea I have ever heard and I would change it on day one.”

But at the same time, the candidates repeatedly struggled to explain how they would handle a problem like Mr. Erdogan, an ostensible ally who has in the past few months bought Russian air defenses and sent troops to seize territory in a neighboring state.

“Turkey is not a U.S. ally," said Senator Bernie Sanders of Vermont, “when they invade another country and engage in mass slaughter.” But he quickly turned the issue back to Mr. Trump, blaming him, not Mr. Erdogan, for the current crisis.

“You tell me what country in the world will trust the word of the president of the United States,” Mr. Sanders said.