Julian Zelizer is a political historian at Princeton University and a fellow at the New America Foundation. He is the author of "The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society."

Donald Trump during Wednesday night’s debate in Las Vegas. (Joe Raedle/European Pressphoto Agency)

Donald Trump has raised alarm over the “rigged election” as the closing argument of his campaign. Of all of the moments that stood out in Wednesday night’s final presidential debate, none equaled his refusal to tell moderator Chris Wallace that he would concede if Hillary Clinton is victorious. “I will look at it at the time. I will keep you in suspense,” Trump said.

This is not the first time that Trump has advanced the idea that American elections are rigged. After Mitt Romney lost to President Obama in 2012, Trump went on a Twitter rampage as he warned his followers that the results were unfair: “We should have a revolution in this country!” he wrote. Another tweet read: “We can’t let this happen. We should march on Washington and stop this travesty.” (He deleted most of the tweets.) As a key part of the birther movement in 2011 and 2012, Trump was contesting the legitimacy of the 2008 election by raising doubts about where Obama was born.

Trump is not the first Republican to make these sorts of arguments. Near the end of the 2008 election, Sen. John McCain (R-Ariz.) — increasingly desperate as the nation’s economic slide and President George W. Bush’s unpopularity made victory as the Republican presidential nominee that year seem impossible — warned that the voter registration group ACORN was trying to “rig” the election. The organization, McCain warned in October of that year, was “on the verge of maybe perpetrating one of the greatest frauds in the history of this country, maybe destroying the fabric of democracy.”

There have also been a number of Republicans who have claimed in the past that the news media was stacked against them. President Richard M. Nixon and his administration famously liked to blast the allegedly liberal bias of the television networks and big city papers as a central reason many Americans did not trust him.

But Trump has taken this theme to a new level. The warnings of a rigged election, with Trump’s alleging that the news media and the entire political system are attempting to elect his opponent is not simply a throwaway line. It has become his main message. Before most votes have been cast, he has questioned the legitimacy of the entire election and electoral system. His followers, many of whom are passionate and devoted to every word that he says, are listening to these warnings and will think, if he loses, that the election has been stolen.

Many dangers can result from this kind of rhetoric. On voting day, fears are growing that Trump supporters might actually intimidate, threaten and harass Americans as they go to exercise their right to vote. Trump’s rhetoric fits with the sentiments of conservatives who have been behind the adoption of voting restrictions in states such as Texas and Wisconsin based on faulty claims of mass voter fraud. Given that Trump and his surrogates have focused their attention on fraud in the inner city and by illegal immigrants who they say pour in just to vote, there are real concerns about  potentially racist and nativist violence. In an era when we should be trying to increase the number of Americans who vote, Trump’s rhetoric could easily create a hostile, chilling environment in certain areas on Nov. 8.

Trump’s campaign could also grow support for the imposition of more voting restrictions that most experts agree have been a disheartening trend in American democracy. One recent investigation of voter fraud found that there were only 31 credible cases out of 1 billion votes. Voter fraud, according to the Brennan Center for Justice, is less likely than getting hit by lightning. Yet these arguments have been the basis of a massive push by conservatives since the 1980s to undercut the Voting Rights Act of 1965.

As Ari Berman and Michael Waldman have recounted in their respective books, “Give Us the Ballot” and “The Fight to Vote,” the new voter restrictions that are making it difficult for people to vote mostly affect disadvantaged and marginalized communities where the stakes of elections are highest. For these communities, Trump’s rhetoric poses incredible risk.

The arguments about legitimacy also will worsen the kind of partisan distrust that already exists in the polarized electorate. We live in an era when party polarization has become so intense that voters can’t bring themselves to trust candidates from opposing views. Indeed, this is Trump’s saving grace: Polarization might create some kind of floor that will prevent his imploding campaign from becoming the kind of electoral wipeout that Lyndon Johnson enjoyed in 1964 against Barry Goldwater.

But the governing process has come to a virtual standstill as a result of polarization. And social scientists have shown that the Republican Party has moved further and further to the right, going into much more extreme places than Democrats. With Trump spending so much time setting up the groundwork to see Clinton as an illegitimate president, there probably will be many more Republican voters who see no reason for their representatives in Congress to compromise on anything with the commander in chief.

As with much of what Trump has done, his campaign lowers the bar for future candidates. If political polarization delivers a Trump loss within reasonable numbers, future Republicans might conclude that this kind of rhetoric can work if it is done in a more polished style. Democrats might harbor enough anger and resentment about how the Republicans tarnished the historic victory of the first female president and respond in-kind.

Trump’s supporters have been arguing that his warnings are nothing new. They compare Trump to Al Gore, who in 2000 took back his concession after he learned that there were serious irregularities in the Florida vote. Yet Gore offered a very bold concession speech after the Supreme Court reached its decision — something hard to imagine Trump doing.

In another controversy about specific instances of voting, the loser (in this case, Richard Nixon) kept quiet about his opinions even as Republicans contested the results of the 1960 election. In an extraordinarily close race in which John F. Kennedy received 113,000 more votes than Nixon and enjoyed a 303-219 electoral vote victory, Nixon was convinced that Kennedy’s team had engaged in fraud in Illinois and Texas. Although Nixon would later boast that he decided that he would not contest the outcome to avoid a “constitutional crisis” and looking like a poor loser, some of his supporters and aides challenged the results with recounts and investigations. The results did not produce enough votes to overturn the election and there was evidence that the Republicans had committed their own chicanery in more conservative parts of Illinois. Unlike Trump, though, Nixon kept his feelings about the election having been stolen to himself, sharing them only with friends and colleagues.

Trump’s comments are thus a historical anomaly, and they do not bode well for the future of the country. His “rigged” rhetoric is not a response to actual problems in the voting process or real evidence of a concerted effort to throw the election. Rather, it seems to be a conscious effort by the nominee of a major party to question the legitimacy of the election with conspiracy theories as a way to mobilize his supporters.

It is important for more Republicans and Democrats to take a stand. Elections are about many things — which party obtains control of the White House and Congress, which policies receive attention, which are put on the back burner, and which constituencies gain power as a result of new leaders. But this time around it might be that something even greater is at stake — the strength and stability of the very process through which our democracy functions.