Donald Trump has upended any plans the Republican Party might have had to resolve its existential crisis on the issue of immigration. Photograph by Charlie Leight / Getty

Sometimes the silliest episodes in politics reveal much larger truths. Consider Donald Trump. Last week, Senator John McCain told me that Trump’s recent anti-immigration rally in Phoenix, Arizona, had “fired up the crazies.” Trump responded by mocking the senator’s academic record—he called him a “dummy”—and then by denigrating McCain’s military service. "He is a war hero because he was captured,” Trump said. “I like people that weren't captured, O.K.? I hate to tell you.”

The back-and-forth, which Amy Davidson wrote about today, will one day register as another blip of the 2016 Presidential campaign. But it highlighted at least two important truths about American politics, the first of which is that Republicans remain trapped by a prisoner’s dilemma on the issue of immigration.

Republicans almost universally acknowledge that, over the long term, their Party needs to increase its appeal to Latinos in order to win the White House. The first step down that path is to talk about immigrants from Latin America in a respectful way and to marginalize prominent politicians who attack immigrants.

However, what is good for the Party over all is not necessarily good for individual candidates. Trump began his campaign for President by suggesting that most immigrants from Mexico were welfare-dependent, job-stealing criminals—some were “rapists,” in fact—who had been coddled by Republicans and Democrats who were too politically correct to tell such hard truths.

While there was widespread condemnation of Trump’s comments from Democrats and many media commentators, Trump’s Republican rivals remained largely silent. (Senator Lindsey Graham was a notable exception.) The obvious reason behind this silence is that many voters in the G.O.P. primaries are sympathetic to Trump on the issue of immigration. Yes, the typical Republican candidate might think that the G.O.P., in general, needs to condemn anti-immigrant bigots. But there’s no benefit for any one candidate to speak up. A look at the polls supports this cynical analysis: much of Trump’s rise, which has been fuelled by his immigration tirades, has come at the expense of his most conservative rivals, who all see their opening in the race to the right of Jeb Bush, the candidate of the Republican establishment.

Unlike the “rapists” remark, however, the remark about McCain’s capture wasn’t a close call. With the exception of Ted Cruz, who seems to be setting himself up as the Trump voter’s second choice, every Republican Presidential candidate rushed to social media to pillory Trump. That, however, may be too little and too late. If the Party’s long-term existential crisis, especially at the Presidential level, is with Latinos, then it not only missed an opportunity to stand with a population being crassly caricatured by a demagogue, it also signalled that attacking immigrants is O.K., while attacking veterans is out of bounds. The condemnation of the "capture" comment, in that sense, then makes the lack of condemnation of the "rapists" comment look even worse.

The second thing the incident showed is that political parties in general, but especially the G.O.P., are becoming impotent. After the 2012 election, the new G.O.P. chairman, Reince Priebus, published a refreshingly honest and relatively unprecedented report about why Mitt Romney lost and how Republicans could win in 2016.

One of his most important findings was that Republicans needed to moderate their language on immigration. But he went beyond recommending an image makeover. He said that Congress should pass “comprehensive immigration reform,” a phrase that has a specific meaning in the context of the legislative debates over immigration: a bill that includes enhanced border security, new temporary-worker programs, and—most controversially—a pathway to citizenship for millions of undocumented immigrants already in the United States.

Trump, who has dominated Republican-primary coverage for more than a month, and who, in a new ABC poll, has registered his largest lead since he entered the race, in June, has been like a Daisy Cutter to this strategy. He has made his rivals look weak and unwilling to speak up for the Latino community, and he has made it even more difficult than usual for them to embrace the one piece of legislation that the R.N.C. believes is essential for the Party’s future.

The R.N.C.’s weakness in the face of Trump’s takeover of the process has been revealed in other ways. In addition to pushing Republicans to embrace immigration reform, Priebus embarked on an effort to make the Presidential primary season less circus-like than it had been in 2012 by keeping down the number of debates and making some adjustments to the primary-voting schedule. Trump has upended those efforts.

Priebus has been powerless. In a phone call early this month, he gently encouraged Trump to tone down his rhetoric, but Trump mocked him and ignored him. In recent days, the R.N.C. has claimed that it can’t do much more. It says that F.E.C. rules prevent it from excluding Trump from the debates, and the R.N.C. has been unwilling to further distance itself publicly from Trump.

This shouldn’t be too surprising. Ever since the nineteen-seventies, when both parties opened up their nominating systems to become more democratic, the parties have been less and less able to exercise control over the messy system of producing a Presidential nominee. Self-funded personalities with no history of working their way up through the ranks have been a feature of almost every campaign.

Even worse for efforts to control the process, the recent Supreme Court decisions on campaign finance have meant that much of the important work of parties has moved over to Super PACs. Earlier this year, before it was dealing with Trump’s kamikaze-like assault on the G.O.P., the R.N.C. was grappling with the Koch brothers’ attempt to essentially set up their own alternative to the Republican Party.

One can laugh at Trump and his absurdities, but it’s wrong to say that he doesn’t matter. He has exposed and exploited the Republican Party’s two great weaknesses: the fact that many of its voters don’t agree with Party leaders on immigration and the fact that the Party is powerless to do much about it.