Republican presidential candidate Donald Trump. (Evan Vucci/Associated Press)

When this election is over, can we acknowledge that at least one big debate in American politics is settled? I am referring to the question of whether our two parties have moved equally far away from the political center, or whether the polarization in our politics is asymmetric.

The very fact of Donald Trump’s nomination should be seen as proof that the Republicans have strayed much further from middle-ground opinion. Advocates of the they’re-both-the-same view should finally throw in the towel.

One party, the Democrats, nominated the mainstream or “establishment” choice: Hillary Clinton. The Republicans chose Trump. In the popular vote in the Democratic primaries, Clinton won 55 percent against Bernie Sanders. In the Republican primary, Trump won 45 percent, while Ted Cruz, nobody’s idea of a middle-of-the-road politician, came in second with 25 percent. In other words, more than two-thirds of the Republicans who voted in primaries cast their ballots for the two candidates who most distanced themselves from moderation.

That should be case closed, and in recent days, new data has come along to show that the Republicans really are way out there when it comes to attitudes toward the political system itself. What is a more basic question than the matter of whether our electoral process is assumed to be on the level? Trump’s insistence before a single vote is counted that the election is “rigged,” and the fact that so many rank-and-file Republicans agree with him, shows how extreme a large chunk of the Republican electorate has become.

The plainest evidence comes from an NBC News/SurveyMonkey poll conducted October 20. Pollsters asked respondents: If their candidate lost the election, would they accept the result? Among Republicans, 45 percent said they would be unlikely to or definitely would not accept the result. Among Democrats, only 16 percent said the same. The gap defines asymmetric polarization.

An ABC News poll released October 23 found that 59 percent “of likely voters … reject Trump’s suggestion that the election is rigged in Clinton’s favor, and more, 65 percent, disapprove of his refusal to say whether he’d accept a Clinton victory as legitimate. Most strongly disapprove, a relatively rare result.”

But the parties did not respond in the same way. When ABC’s analysts broke down the results by party, they found that only 23 percent of Republican likely voters “say [Trump is] trying to make excuses in case he loses, rather than raising a legitimate concern; this view swells to 57 percent among independents and 91 percent among Democrats.” Again: asymmetry.

I am not neutral here. I have long insisted that the polarization in our politics is asymmetric and have championed the pioneering work of my friends and colleagues Thomas E. Mann and Norman J. Ornstein.

In my book “Why the Right Went Wrong,” I cited data showing that, while 59 percent of Democrats preferred politicians who “make compromises with people they disagree with” over those who “stick to their positions,” only 36 percent of Republicans made the same choice. I also noted that there far more Republicans who call themselves conservative than there are Democrats who call themselves liberal.

Mann and Ornstein made the definitive case in their 2012 book “It’s Even Worse than It Looks,” in which they bravely challenged the easy “both parties are the same” assumption. Here is a summary of their view from an article they published in The Post’s Outlook section:

“The GOP has become an insurgent outlier in American politics. It is ideologically extreme; scornful of compromise; unmoved by conventional understanding of facts, evidence and science; and dismissive of the legitimacy of its political opposition. When one party moves this far from the mainstream, it makes it nearly impossible for the political system to deal constructively with the country’s challenges.”

When you think about it, this is a rather good description of what Trump has done to the 2016 presidential campaign.

Let it be emphasized that no one denies Democrats are more liberal now than they were when the party included Southern segregationists before the civil rights era. And the proportion of Democrats who identify as liberal is on the rise, partly because millennial voters constitute the most progressive generation since the FDR-era cohort and are a growing part of the Democratic coalition. But it needs to be stressed again that Republicans are still far more uniformly conservative than Democrats are liberal — and as the Trump experience shows, far more likely to gravitate toward extreme positions.

After this election, Republicans need to face up to what the rise of Trump means for their future. But political analysts need to take a lesson from the Trump experience, too. It’s not partisan to say that polarization is asymmetric. If you are willing to look at the evidence provided by Republican voters themselves, you have to conclude that it is a simple description of fact.