Elizabeth Warren, a Democrat, represents Massachusetts in the U.S. Senate.

Cratering in the polls, besieged by sexual assault allegations and drowning in his own disgusting rhetoric, Donald Trump has been reduced to hollering that November’s election is “rigged” against him. His proof? It looks like he’s going to lose.

Senior Republican leaders are scrambling to distance themselves from this dangerous claim. But Trump’s argument didn’t spring from nowhere. It’s just one more symptom of a long-running effort by Republicans to delegitimize Democratic voters, appointees and leaders. For years, this disease has infected our politics. It cannot be cured until Republican leaders rethink their approach to modern politics.

Anyone with children knows that whining about imaginary cheating is the last refuge of the sore loser. But GOP leaders have served up such a steady diet of stories about imaginary cheating that an Economist-YouGov poll shows that 45 percent of Republican voters believe voter fraud is a “very serious problem,” and 46 percent have little or no confidence that ballots will be counted accurately. They hold these views even though there is literally no evidence — none, zero, zip — that widespread voter fraud is a factor in modern American elections. A recent study looked at around a billion ballots cast in the United States from 2000 through 2014 and found only 31 instances of impersonation fraud at the polls.

Sen. Elizabeth Warren (D-Mass.) gave an impassioned speech for Hillary Clinton in Denver Oct. 16, and said Donald Trump's crude remarks about women don't just make her "sick," they make her "furious." (Reuters)

Republican leaders — and even Trump’s running mate — have tried to tiptoe out of the room when Trump makes ever-wilder claims of a rigged election. But as much as these Republicans would like everyone to believe that this is a Trump-only problem, it’s not.

For years, Republican leaders have pushed the lie that voter fraud is a huge issue. In such states as Kansas and North Carolina , and across the airwaves of right-wing talk radio and Fox News, Republican voters have been fed exaggerated and imagined stories about fraud. Interestingly, all that fraud seems to plague only urban neighborhoods, minority communities, college campuses and other places where large numbers of people might vote for Democrats. The purpose of this manufactured hysteria is obvious: to delegitimize Democratic voters and justify Republican efforts to suppress their votes.

[Trump supporters are talking about civil war. Could a loss provide the spark?]

The voting-fraud lie has been used to justify the passage of dozens of voter ID laws, typically rammed through state legislatures by Republican partisans. A study by political scientists at the University of California at San Diego recently concluded that strict photo-identification requirements disproportionately suppress turnout by Democratic voters — especially blacks and Latinos. Meanwhile, after a key provision in the Voting Rights Act protecting minority voters from discrimination was unceremoniously declared defective by a right-wing majority on the Supreme Court in 2013, those same Republican leaders who seem so concerned about threats to the integrity of our elections have largely remained on the sidelines.

Trump also didn’t invent ominous appeals for partisans to patrol “certain areas” and “go and watch these polling places” where citizens often vote for Democrats. More than three decades ago, the Republican National Committee was caught orchestrating expansive efforts to intimidate individuals at polling places in minority neighborhoods. Federal courts have barred the RNC from engaging in poll-watching activities relating to “ballot integrity, ballot security or other efforts to prevent or remedy vote fraud” in minority areas ever since.

It’s not just voters, either. Trump’s effort to delegitimize federal officials and political opponents also shares a long-standing Republican pedigree.

After Trump was sued for fraud over Trump University, he attacked the legitimacy of the federal judge with Mexican heritage presiding over the case, claiming that Trump’s own bigotry undermined the judge’s neutrality. Paul Ryan tsk-tsked, but Trump was simply joining a long line of Republicans in Congress who have spent years assaulting the federal courts. For years, the Republicans have blocked scores of nonpolitical lower-court nominees who haven’t pledged their allegiance to the financial interests of the rich and powerful. These attacks culminated in a national campaign of slime against the president’s highly respected choice to fill a vacant Supreme Court seat. It’s no surprise Trump would conclude that federal judges are fair game.

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[Trump is rigging the election: No matter who wins, America loses]

Similarly, some Republicans pretended to be shocked when Trump asserted that he would follow two-bit tyrants such as Russian President Vladimir Putin and Iran’s Ayatollah Ali Khamenei and imprison his political rival after the election. But for years, congressional Republicans have focused most of their resources on finding some way to brand Hillary Clinton a criminal. A party that wastes millions of taxpayer dollars on eight separate Benghazi investigations — and shouts itself hoarse attacking an FBI director who served as a senior political appointee in a Republican administration when he concludes that no reasonable prosecutor would bring charges against Clinton over her emails — shouldn’t feign astonishment when its presidential nominee echoes their efforts to criminalize American politics.

Democrats and Republicans disagree about a lot of issues. We both fight hard to win elections. But winning isn’t everything. Al Gore understood that when he stood down after the 2000 election. Now Republican leaders seem increasingly concerned that when Trump loses, he won’t follow that example. But Trump’s words and deeds are merely the latest — and loudest — examples in a long line of Republican tactics that are poisoning our political system.