OPINION — Most polls show any of a half-dozen Democratic candidates leading Donald Trump in the 2020 presidential contest. But only Bernie Sanders (and possibly Elizabeth Warren) would change the general election from a referendum on the president into a choice between two general election nominees.

Whatever you think about Sanders — whether you think he’s an anti-establishment voice for change who promises to help the ignored and repressed, or an ideological demagogue who failed to understand the nature of the Soviet and Cuban communist regimes — there should be little doubt that the Vermont senator is a controversial, even divisive, figure.

His repeated embrace of socialism, attacks on millionaires and billionaires, contempt for corporate America, and support for free college, “Medicare for All” and deep cuts in defense spending surely place him at the fringes of the American political mainstream — if not outside it.

Again, you may think that Sanders’ prescriptions for change are necessary and will produce a more just society. But it would be naive not to see them as controversial, even within Democratic ranks.

So instead of a November matchup between Trump and Sanders being “about” the man currently in the White House — about his vulgarity, his profanity, his lies, his efforts to undermine key institutions (like the FBI, the CIA and a free press), his efforts to undercut the power of Congress and his anger and divisiveness — suddenly a 2020 general election becomes as much “about” Bernie Sanders as it would be “about” Donald Trump.

Why is that risky for Democrats?

It’s risky because instead of broadening the potential coalition against Trump by uniting Democrats, independents and even a handful of Republicans offended by the president’s behavior, November’s general election would become a choice between two deeply flawed politicians.

Sanders is as angry as Trump. His supporters are as nasty as Trump’s.

Like Trump, Sanders condemns the establishment and treats the wealthy as if they have no social conscience and have done nothing to advance the economy.

He has embraced a label, socialism, that will be off-putting to many who might otherwise support him as an alternative to Trump.

Both Trump and Sanders are “base first” politicians who see little reason to compromise, negotiate or reach out to those not already in their coalition. The folks in the middle — and those who for whatever reason find Trump unacceptable — now have a choice between two unappealing loud-mouths, not a referendum on an out-of-control president.

More than a year ago, in a Nov. 19, 2018 column published shortly after the House Democrats’ dramatic midterm victory, I predicted Democrats would spend more than a year fighting among themselves about their party’s presidential nomination:

“Progressive and pragmatist/establishment Democrats will demonize each other during debates and campaign stops, making it more difficult for the party to rally behind the eventual nominee.

“That fight will give Trump many opportunities to belittle his adversaries, play up Democratic divisions and remind Republicans, independents and college-educated white women why they didn’t vote Democratic in 2016.”

I believe that is exactly what has happened. A Sanders nomination would give more ammunition to the president, increasing the likelihood that the 2020 contest would turn into a referendum on Sanders.

College-educated whites (especially women, but also men), the key swing group in 2018, might ultimately vote for Sanders, or they might find him unacceptable and swing back to Trump. And that could produce another Trump win in Pennsylvania, Michigan, Wisconsin and Florida, in addition to Minnesota and New Hampshire.

I know the Sanders response. He’ll talk about a cross-generational, biracial coalition that will upend American politics. And maybe that will happen.

Maybe young voters will finally turn out in huge numbers instead of being all talk and little vote. Maybe black voters will come out in numbers like they did in 2008 rather than like they did in 2016. And maybe Cuban and Central American immigrants in Florida will warm to a nominee who embraced Fidel Castro and Daniel Ortega.

The easiest way for a Democrat to defeat Trump is to make the election about the president. That could galvanize a wide range of the president’s critics who might not agree with each other on other matters.

That doesn’t mean that Sanders cannot win following his own strategy. I don’t know whether he can.

But Democrats potentially can have a much larger coalition if they nominate someone who makes Trump the central issue in the election, not their own nominee.