Democrats and Republicans seem to operate under differing rules of political combat … in this presidential election cycle, at least.

Republicans opened the presidential primary campaign with 17 individuals seeking their party’s nomination. One of them remains. He is likely the most improbable candidate you ever could imagine.

Donald J. Trump is a man with zero public service record, a scatter-shot approach to what passes as foreign and/or domestic “policy” and a checkered personal history.

He’s the last man standing among all those Republicans.

Democrats opened their season with just five candidates. Three of them are now off the grid. Two are left: U.S. Sen. Bernie Sanders and former Secretary of State Hillary Rodham Clinton.

Clinton’s all but got her party’s nomination in the bag. Sanders is hanging on, cheered on by those big rallies.

So, here’s what I believe should happen: Sanders needs to call it a campaign. He’s made his point — repeatedly — about income inequality and Wall Street corruption. He’s not going to be nominated president.

It’s time for him to clear the field for Clinton to run against Trump — head to head.

Democratic gurus are growing a bit restive. They see these polls that show Clinton and Trump in a close race. They fear that the longer Sanders continues his sniping at Clinton, the more damage he inflicts on her chances to become the nation’s 45th president.

My own view is that this contest shouldn’t even be close.

Trump is patently — at virtually every level one can name — unfit to become president. Yet he continues to win cheers from those who think he “tells is it like it is.” They rally to his calls against what he calls “political correctness.” The man is a buffoon … yes, a wealthy one, but a buffoon nonetheless.

Clinton is far from the perfect candidate. But she’s been examined up close and personal for more than two decades. Her career — as first lady, U.S. senator and secretary of state — has been dissected more carefully than a laboratory frog.

She continues to fend off the challenge from the remaining other Democrat in this contest.

The primary season is over, Sen. Sanders. You lost. Hillary Clinton won.

It’s time for Sen. Sanders to “suspend” his campaign and then start writing the fiery speech he plans to give at the Democratic Party’s presidential nominating convention this summer in Philadelphia.

As for Trump … well, uh, keep doing what you’re doing.