In every contested election there are winners and losers. While lots of attention, money, and power flows to the winners, it is really the losers who are key to keeping democracy healthy.

More specifically, it is important that the people who lose elections recognize that they lost fair and square — assuming that that is indeed the case — and that they convey that truth to their supporters.

It turns out that learning to be a good loser is not just a life lesson for children playing board games; it is also an important lesson for politicians. More than that, graceful concessions by losing candidates constitute a sort of glue that holds the polity together, providing a cohesion that is lacking in less-well-established democracies.

By talking preemptively about a "rigged system" that may well lead to a "stolen election," Donald Trump is weakening his voters’ faith in elections in a way that could prove extremely dangerous.

Of course, no one likes losing elections: not candidates, and not the people who vote for them. Take the evidence in the chart below, which comes from the American National Election Study’s post-election polling data in 2012.

In the weeks after the election, respondents were asked a series of questions about their faith in politics, politicians, and elections — for instance, "How much say do you have in government?" and "Does it make a different who one votes for?" — and instructed to answer with a number from 1 to 6. The higher the number, the more the respondents think elections are fair, that votes are counted fairly, and the happier they are with democracy. In the chart, the different bars distinguish between those who voted for the winning candidate for president in 2012 (Obama) and those who voted for a losing candidate (typically Romney).

There are three points to make about this data. First, the gap between winners and losers is seen across a whole range of survey questions: It is not simply one response to one question but a whole bundle of responses. (And all of these differences are statistically significant.) Second, and more consequential, the responses cannot be interpreted as, or dismissed as, straightforward expressions of disappointment. The survey questions — whether politicians listen to people, and so on — tap into fundamental properties of democratic practice.

Collectively, the responses show that when voters see their candidate lose, their regard for the system of government as a whole takes a hit. Even within an established democracy and after an election that was not especially controversial, we see weakening legitimacy of the system in the eyes of losers.

Third, comparative data demonstrates that the gap between winners and losers is not just a property of this particular election or just of US elections, but one that is commonly seen in elections around the world: Winners and losers see the outcome in quite different terms.

Ordinarily, those kinds of reactions fade. One of the mechanisms that helps is that, at some future election, losers become winners and their faith is restored.

But it is clear that, in the aftermath of a loss, there is plenty of kindling for irresponsible politicians to set fire to. They could stoke the feelings that remain temporary in ordinary times, transforming them into civic unrest and even civil disobedience. Most politicians who lose elections recognize this potential for mischief, and so they ordinarily make a creditable run at helping to keep matters calm.

A textbook example is provided by President George Bush Sr., whose concession speech included the following statement: "Here's the way we see it and the country should see it — that the people have spoken and we respect the majesty of the democratic system. I just called Gov. Clinton over in Little Rock and offered my congratulations. He did run a strong campaign."

In making this statement, President Bush signaled that the election was over and he lost fair and square.

Remember, the lowered levels of regard for the system displayed in the chart occurred in an election in which no candidate was crying foul. Crying foul widens the gap in opinions, undermining the support for and legitimacy of the system as a whole.

We have seen this in recent elections in Mexico, Ukraine, and Georgia. The consequence has been popular unrest and disturbances. One could argue that several of those elections were, in fact, "dirty," and so people were right to protest at an election being stolen. (Genuine corruption is another demonstrable way to undermine faith in democracy.) But problems come when the election was clean and yet the losing politician refuses to accept the result was arrived at fairly.

This year in the US we have seen two examples of the more worrisome kind of candidate behavior. Early this year, Bernie Sanders claimed that the series of primary elections — in which he won more than 3.5 million fewer votes than Hillary Clinton — was rigged. The narrow practical implication of such claims was to weaken support for Clinton among Democrats and make it less likely that Sanders’s supporters would rally behind her. The broader consequence was an erosion of the legitimacy of the system.

Donald Trump is a candidate who seems to want to claim that, whenever he loses, it is not because more people voted against him than for him, but because the election is rigged. From this assumption arises his frequent call for "poll watchers" with no legal authority and questionable roles. (Will they intimidate voters? Will they interfere with the process?) The longer-term consequence will be the same as in Mexico, Ukraine and elsewhere: They will undermine voter support for the system. The difference is that in the US, the claims have very little basis in actual levels of election fraud.

In acknowledging their loss as fair, candidates helped reconcile their supporters to loss and a peaceful change in power. Comments like those by President Bush reinforce the idea that in democracy losing is something that happens — fair and square. Such comments by political leaders strengthen our democracy.

So — after the celebration parties of the winners, after the balloons and bands, after the champagne and confetti — spare a thought for the losers in the coming election. At least, spare one for the losers who model their responses on President Bush in 1992. We need their restraint in order to keep our democracy.

Shaun Bowler is a professor of political science at the University of California Riverside and a co-author of Losers’ Consent: Elections and Democratic Legitimacy.