The Republican establishment profited from the insinuations that Obama is a Muslim, that he’s anti-American, that his health care plan would lead to “death panels.” Rick Perry has described Trump as a “cancer on conservatism” and said his movement is “a toxic mix of demagoguery and meanspiritedness and nonsense that will lead the Republican Party to perdition” — indeed, but it was a mix that too many Republican leaders accepted as long as it worked for them.

This echo chamber deluded its believers to the point that it sometimes apparently killed them. During the 2009-10 flu pandemic, right-wing broadcasters like Limbaugh and Glenn Beck denounced the call for flu shots, apparently seeing it as a nefarious Obama plot.

The upshot was that Democrats were 50 percent more likely than Republicans to say that they would get flu shots, according to a peer-reviewed article in The Journal of Health Politics, Policy and Law. So when the pandemic killed up to 18,000 Americans, they presumably were disproportionately conservatives.

The Republican strategy also nurtured expectations at the grass roots that could never be met. “The Republican Party created Donald Trump,” said Erick Erickson, the conservative radio host, “because they made a lot of promises to their base and never kept them.”

This is a theme of a smart new book by E.J. Dionne Jr., “Why the Right Went Wrong,” who argues that Republican leaders repeatedly made unrealistic pledges — of smaller government, preservation of bygone values and an end to demographic change. “The history of contemporary American conservatism is a story of disappointment and betrayal,” he writes, and that helps explain the disenchantment with the Republican establishment.

Maybe Trump’s campaign will fall apart, but he has a huge lead in the polls in the South Carolina primary coming up, and he has already done enormous damage to the G.O.P. establishment.

So today the leading candidate for president in the party of Lincoln is an ill-informed, inexperienced, bigoted, sexist xenophobe. And he’s not a conservative at heart, just a pandering opportunist.

Donald Trump is the consequence of irresponsible politicking by Republican leaders, the culmination of decades of cultivating unrealistic expectations within the politics of resentment. It’s good to see leading Republicans standing up to him today, but the situation recalls the Chinese saying, qi hu nan xia — when you’re riding a tiger, the hard part is getting off.