Mr. Ryan and Mr. McConnell could hardly dishonor themselves more than they already have in this sordid election year, but their refusal to stand up to Mr. Trump’s pernicious lie may be their lowest moment yet.

Other high-profile Republicans have amplified Mr. Trump’s charges and further riled up his angry base. On Saturday, Senator Jeff Sessions, a member of the Senate Judiciary Committee from Alabama, told a crowd at a Trump rally in New Hampshire that “they are attempting to rig this election.” On Sunday, Rudy Giuliani, the former New York City mayor and now Mr. Trump’s race-baiting surrogate, told CNN that he would be a “moron” to believe that the voting in cities like Chicago and Philadelphia would be fair to Mr. Trump. “I have found very few situations where Republicans cheat,” Mr. Giuliani said. “They don’t control the inner cities the way Democrats do.”

So what should patriotic Republicans be telling the nation?

First, they must reject bogus claims of voter fraud, which has been shown time and again to be virtually nonexistent. That will take work, since the specter of fraud, inevitably tinged with racist overtones, has long been a rallying cry among Republicans. But never before has a major party’s presidential candidate openly encouraged this attack on the legitimacy of the electoral process. Mr. Trump does it almost daily. On Monday morning, he tweeted, “Of course there is large scale voter fraud happening on and before Election Day.” His claims are never backed up by any evidence, but they’re still working: Only about one-third of Trump supporters believe their vote will be counted accurately, in contrast with two-thirds of Hillary Clinton’s supporters, according to a Pew Research poll.

Second, top Republicans must reject the ridiculous notion that a national election can be “rigged.” Voting is, by design, a decentralized process run by local elections officials of both major parties. With his irresponsible language, Mr. Trump is calling into doubt the integrity of Republican and Democratic officials everywhere. Some have pushed back, like Jon Husted, Ohio’s Republican secretary of state, who called it “wrong and engaging in irresponsible rhetoric” to throw around evidence-free allegations about vote-rigging. Representative Peter King, a Republican from New York and Trump supporter, said on Monday: “Is it legally rigged? No, it’s not. Whoever wins, wins.”

Trump apologists claim that when he says the election is rigged, he is only referring to critical media coverage, but that’s demonstrably false. Either way, some of his supporters have swallowed his lies and are threatening to act as vigilante poll watchers on Election Day. One Trump supporter in Ohio told The Boston Globe that he would look for “Mexicans. Syrians. People who can’t speak American” and that he would “go right up behind them” and “make them a little bit nervous.”