Someone needs to tell the Democrats running for president to watch Independence Day. Or maybe brush up on their history of World War II.

See, there’s this concept of banding together against a common enemy that they just don’t seem to understand.

For all the hand-wringing about Donald Trump and how he’s changed the game — and the stakes — in this year’s presidential election, Democrats this primary season are operating as though it’s business as usual.

Instead of focusing their fire on that common enemy, they’ve circled the wagons ... and set their sights on one another. Joe Biden mocks Pete Buttigieg’s inexperience. Buttigieg, Biden, Bloomberg and Klobuchar seek to undermine Bernie Sanders by saying he can’t get elected (even though polls show him running just as well, or better, against Trump than his Democratic rivals).

Candidates spar over universal healthcare, minority rights and other issues — all worthy considerations. But instead of targeting Trump, with whom they vehemently disagree, they’re nitpicking each one another to death. They’re so concerned that maybe “a socialist can’t beat Trump” or “a woman can’t beat Trump” or “a gay man can’t beat Trump” or “a mayor can’t beat Trump” or “an old guy can’t beat Trump” that they’re trying to kill each other off with elephant guns that will be out of ammunition by the time November rolls around.

By that time, Trump will have all these sound bites showing Democrats blasting each other other, and he’ll use them against whoever wins the nomination.

My father, a political science professor, called this phenomenon, “Your own guy says so.” If Buttigieg says Sanders’ talk of universal healthcare is dangerous, Trump can use that. If Biden says Buttigieg lacks the experience to be president, he’ll use that, too. It’s all right there on videotape.

The complicit media

The media, of course, feed into all this, not because it’s in the public interest, but because bare-knuckle brawls make good theater (and ratings!). Forget all the hoopla about CNN or MSNBC carrying water for the Democrats. They’re not out to get Democrats elected. They’re out for ratings ... which is, in fact, the same reason Fox echoes Trumpian talking points. They don’t care about Trump. Not really. They care about their bottom line. Viewers tune in, advertisers buy spots, the network makes money.

Anyone who thinks any network’s foremost mission is to elect this or that candidate is fooling themselves. They’re in it for the Benjamins, plain and simple.

Which is why CNN and MSNBC are helping to destroy the Democrats’ chances of winning, whether they’re willing to admit it or not. Four more years of Trump is the best thing that could happen to them. It perpetuates outrage, which perpetuates viewers, which perpetuates ratings, which keeps the cash flowing.

All those town halls and debates aren’t any kind of public service. They’re aired for the same reason pay-per-view is showing the Wilder-Fury heavyweight rematch this weekend. For the same reason car crashes and fires lead the local evening news. Conflict sells. Brutality raises ratings. They don’t want to see Elizabeth Warren try to play peacemaker among her fellow Democrats, or hear Sanders say he doesn’t care about Hillary Clinton’s damn emails. They want to see Democrats going for the throat. They want to see an embittered Clinton lashing out at Sanders four years after the fact, and they want to see Sanders react.

They absolutely loved the Iowa caucus debacle, because viewers stayed tuned to find out the delayed results, and because it created still more conflict. Then they could pontificate and hand-wring as though they’re above it all. Yeah, right.

And they don’t want Trump out of office. They love the guy, because he’s the archvillain everyone (on the left, anyway) loves to hate. Who are the Avengers without a Thanos? Who’s Batman without the Joker? Trump knows they need him, and he goads them with it, and the Democrats respond with...

Business as usual. Distract one another from the real opponent by getting into a family food fight, while the other guy goes around selling bogus promises of filet mignon dinners at the steakhouse down the street — which is really just a front for a two-bit mob operation.

Democrats are fiddling around while what used to be America burns. The media bring it all right into our living rooms. And Trump’s just laughing all the way to a second term.