Republican presidential nominee Donald Trump speaks at a campaign rally on Sept. 13 in Clive, Iowa. (Steve Pope/Getty Images)

WHAT DONALD TRUMP popularized as a Big Lie — the birther myth about President Obama — is now a shibboleth among his followers and many Republicans. It matters not a whit that Mr. Trump has finally, for blatant political purposes, admitted that the president was born in the United States; large numbers of his partisans, and of Republicans generally, still don’t believe Mr. Obama has a legitimate claim to the office he has held for nearly eight years.

Birtherism, a hoax perpetrated on Americans, is proof positive of the enduring efficacy of the Big Lie, the proposition that people will sooner believe a monumental falsehood than a trivial one, especially if it is repeated often enough. The cost of such a hoax is not only to the truth but also to the democratic process, which is rendered ridiculous by the ensuing debate. Mr. Trump has revealed his own facility with fraud and deceit, and he has also exposed how vulnerable democracy is when confronted with a charlatan-celebrity, bereft of principles and willing to say anything to grab headlines.

The cancer of corruption perpetuated by Mr. Trump’s dazzling dishonesty has infected not only his campaign but also the Republican Party, which falls in line, sheeplike, to defend his every lie.

Now Mr. Trump says falsely that Hillary Clinton was the originator of birtherism? GOP officials say so, too. Now Mr. Trump claims credit for putting to rest an “issue” he himself perpetuated? GOP officials say so, too. No pronouncement is too preposterous for Republican National Committee Chairman Reince Priebus and the party’s other unscrupulous grandees.

Starting in the spring of 2011, as he weighed a possible presidential campaign in 2012, Mr. Trump appropriated what had been a fringe conspiracy theory, claiming that “I have people” who had investigated the provenance of Mr. Obama’s birth certificate, and “they cannot believe what they’re finding.” Later Mr. Trump suggested the president’s supposed motive for not releasing his so-called long-form birth certificate — he had already released an official Hawaii certification of live birth — was to conceal his religion. “Maybe it says he’s a Muslim,” he said.

Birth certificates do not classify people by religion, and the president is Christian. No matter: Mr. Trump kept up his birther blather, saying that it “made me very popular,” and helping to revive the sagging ratings of his TV show “The Celebrity Apprentice.”

Now, having clarified that Mr. Obama was, in fact, born in the United States, Mr. Trump could render a further service by retracting various other frauds he has perpetrated in the course of this campaign. He could start by acknowledging there is no evidence he opposed the Iraq War before it started in March 2003, as he has claimed repeatedly. He could drop the ludicrous suggestion that Mr. Obama’s best-selling autobiography, “Dreams From My Father,” was written by the onetime radical activist Bill Ayers. He could take back his racially incendiary and wildly inaccurate assertion that 81 percent of white homicide victims are killed by blacks. And that’s just for starters.