WASHINGTON — The fixed smile on Donald Trump’s face as Sarah Palin unleashed her free-association, who-knows-what-she’ll-say-next harangue endorsing him on Tuesday sent its own message. “How long do I have to stand here?” it seemed to say. But of all the developments in the astonishing Republican presidential contest, this moment told us what we need to know about the state of a once-great political party.

Consider the forces that brought Palin to the national stage in the first place. In 2008, John McCain, running behind Barack Obama in the polls, wanted to shake up the contest by picking a moderate as his running mate. His first choice was Sen. Joe Lieberman, and he also liked former Pennsylvania Gov. Tom Ridge.

But McCain won the nomination against the will of the Republican right as more conservative candidates had fractured their side’s vote. “He is not the choice of conservatives, as opposed to the choice of the Republican establishment — and that distinction is key,” said Rush Limbaugh. The establishment, Limbaugh charged, had “long sought to rid the party of conservative influence.”

A moderate VP choice would have been too much for Limbaugh’s legions. So McCain, facing a full-scale revolt on the floor of the Republican convention, gave up on Lieberman and Ridge, turning instead to Palin.

After Obama won, the main goal of Republican leaders of all stripes was to take back Congress as a prelude to defeating the president in 2012. The angry grass-roots right — it has been there for decades but cleverly rebranded itself as the tea party in 2009 — would be central in driving the midterm voters the GOP would need to the polls. Since no one was better at rousing them than Palin, old-line Republican leaders embraced and legitimized her even if they snickered privately about who she was and how she said things.

Today’s Republican crisis was thus engineered by the party leadership’s step-by-step capitulation to a politics of unreason, a policy of silence toward the most extreme and wild charges against Obama, and a lifting up of resentment and anger over policy and ideas as the party’s lodestars.

Many Republicans are now alarmed that their choice may come down to Trump, the candidate of a reality-show populism that tries to look like the real thing, and Sen. Ted Cruz, an ideologue whom they fear would lead their cause to a devastating defeat. There is an honorable pushback against this outcome from champions of a genuinely more moderate and tolerant brand of conservatism — the columnists Michael Gerson and David Brooks among them.

But this is a battle that needed to be joined long ago (which, I should say, is a central theme of my new book, “Why the Right Went Wrong”). A showdown was required before the steady, large-scale defection of moderate voters from the party. Now that opponents of Trump and Cruz need the moderates, they are no longer there — except, perhaps, in states where independents might cross into the party’s primaries to save it from itself.

And instead of battling the impulses now engulfing the party, GOP honchos exploited them. They fanned nativist feeling by claiming that illegal immigrants were flooding across our borders, even when net immigration from Mexico had fallen below zero.

They promised radical reductions in the size of government, knowing no Republican president, including Ronald Reagan, could pull this off. They pledged to “take the country back,” leaving vague the identity of the people (other than Obama) from whom it was to be reclaimed. Their audiences filled in the blank. They denounced Obamacare as socialist, something, as Sen. Bernie Sanders is pointing out, it decidedly is not. Indeed, it’s rooted in proposals Republicans once made themselves.

Politicians whose rhetoric brought the right’s loyalists to a boiling point now complain that they don’t much like the result. But it’s a little late for that. Why shouldn’t the party’s ultra-conservatives and its economically distressed working-class supporters feel betrayed? At least with Trump, Cruz and Palin, they have reason to think they know what they’re getting. “We are mad and we’ve been had,” Palin declared on Tuesday. “They need to get used to it.”

So watch for the establishment’s next capitulation. There are reports that some in its ranks are already cozying up to Trump. Given the record, there’s little reason to doubt this.

E.J. Dionne Jr. is a columnist for The Washington Post. His email address is: ejdionne@washpost.com. Twitter: @EJDionne.