In 2008, Barack Obama won the presidency promising that he would heal our political divisions. Instead, Mr. Obama has been as polarizing as any president in the history of modern polling. The debate over the Syrian refugee crisis illustrates why.

The civil war in Syria has created one of the worst refugee crises since World War II, and the president has instructed his administration to admit at least 10,000 refugees in fiscal year 2016. Republicans in Congress, in the aftermath of the massacre in Paris on Nov. 13, called for a pause in this process, in part because of their fear that terrorists might pose as refugees. The president, rather than trying to persuade his critics, mocked them.

“Apparently they’re scared of widows and orphans coming in to the United States of America as part of our tradition of compassion,” Mr. Obama said. “That doesn’t sound very tough to me.” According to the president, the most potent recruitment tool for the Islamic State isn’t jihadist social media or battlefield victories but Republican rhetoric. “They’ve been playing on fear in order to try to score political points or to advance their campaigns,” he said.

The president flippantly dismissed worries about the vetting process despite the fact that, as James R. Clapper Jr., the director of national intelligence, said in September, the possibility that the Islamic State might infiltrate operatives among Syrian refugees is “a huge concern of ours.”

Administration officials also acknowledge that there are limitations on determining the history of Syrian refugees since we’re in no position to collect vital information from Syria. Even if the president believes the case for accepting refugees overrides those concerns (as I basically do), he should acknowledge their legitimacy.

What made Mr. Obama’s assault on Republicans particularly outrageous is his hypocrisy, by which I mean the president’s failure to act in any meaningful way to avert the humanitarian disaster now engulfing Syria. It’s not as if options weren’t available to him.

In 2012 Mr. Obama rebuffed plans to arm Syrian rebels despite the fact that his former secretaries of defense and state, his C.I.A. director and the chairman of the Joint Chiefs of Staff supported them. He repeatedly insisted he would not put American soldiers in Syria or pursue a prolonged air campaign. He refused to declare safe havens or no-fly zones. And it was also in 2012 that Mr. Obama warned the Syrian president, Bashar al-Assad, that using chemical weapons would cross a “red line.” Yet when Mr. Assad did just that, Mr. Obama did nothing.

The president, perhaps fearful of offending the pro-Assad Iranian government with which he was trying to negotiate a nuclear arms deal, chose to sit by while a humanitarian catastrophe unfolded. As Walter Russell Mead wrote in The American Interest, “This crisis is in large part the direct consequence of President Obama’s decision to stand aside and watch Syria burn.” Some of us find it a bit nervy for the president to lecture the opposition party for heartlessness because cleaning up after his failure raises security concerns.

A reasonable approach would take account of both the humanitarian crisis in Syria and the concerns of critics of the president’s proposal. Doing so might result in a pause in the process to reassess our security procedures, make improvements where necessary and then proceed. Under the leadership of the new speaker, Paul Ryan, the House has passed just such a proposal with a broad bipartisan majority — 47 Democrats sided with Republicans — but Mr. Obama has promised to veto it if it passes the Senate. In his Manichaean conception of politics, such balance has no place, it seems.

What we have seen and heard from Mr. Obama during the Syrian crisis — self-righteousness without self-reflection, taunting, exasperation that others don’t see the world just as he does, the inability to work constructively with his opponents — have been hallmarks of his presidency. The man who promised to strengthen our political culture has further disabled it.

The president doesn’t bear full responsibility for the fractured state of our politics. The causes are complicated. They predate the Obama presidency, and Republicans have certainly played a role. (For some on the right, compromise is in principle capitulation.)

Yet it was Barack Obama who in 2008 wanted us to “rediscover our bonds to each other” and put an end to the “constant petty bickering that’s come to characterize our politics.” He utterly failed in that and has to own his part in it. According to a new Pew Research Center study, 79 percent of Americans view the country as more politically divided than in the past.

Today our political discourse barely allows us to think clearly about, let alone rise to meet, the enormous challenges we face at home and abroad. Trust in government has reached one of its lowest levels in the past half-century. Americans are deeply cynical about the entire political enterprise; they are losing faith in the normal democratic process.

This creates the conditions for the rise of demagogues, of people who excel at inflaming tensions. Enter Donald J. Trump, who delights in tearing down the last remaining guardrails in our political culture.

Mr. Obama is hardly responsible for Mr. Trump, and it’s up to my fellow Republican primary voters to repudiate his malignant candidacy. Not doing so would be a moral indictment of our party. But in amplifying some of the worst tendencies in our politics, Mr. Obama helped make the rise of Mr. Trump possible.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a contributing opinion writer.