Tonight’s Democratic presidential debate is about one thing and one thing only: Joe Biden and his general fitness.

Oh, sure, Elizabeth Warren will attack rich people, and Bernie Sanders will attack rich people, and they will both promise to spend trillions of dollars they wouldn’t be able to commandeer if either became president.

And Kamala Harris will say something, and Cory Booker will say something else, and Beto O’Rourke will probably curse, and Andrew Yang is supposedly going to do something no one has ever done before, and, from his undisclosed location, Bill de Blasio will ask people to go to his new website, payattentiontome.com.

Who cares?

The only thing that matters tonight is Biden’s performance. He seems to have slipped a little in the polls this week because stories about him confusing Vermont with New Hampshire made him look gaga and him saying he was always against the Iraq War made him look like a liar.

For three hours, Biden needs not to be gaga and not to lie.

Well, maybe he can lie. People don’t seem to care that much about lying any longer. After all, Warren just said she was going to increase Social Security payments by $2,400 by levying an unconstitutional tax she could never get past Congress anyway, so what’s a lie between friends?

It’s the gaga thing what will kill him — if he is, in fact, gaga.

If you went around talking for hours every day and had someone watching your every word, you too could confuse Vermont and New Hampshire, trust me. They both have cows and opioids.

He’s also Joe Biden. This is a man who has spoken more words aloud in his life than Scheherazade, and she was blathering on to save herself from execution.

He talks because apparently he needs to speak words to breathe.

He’d be agony to have as president — first-term Sen. Barack Obama once passed someone a note that said “shoot me now” as Biden was delivering one of his patented blatherfests on the Senate floor — but that has nothing to do with his age.

And yet there is his age. He’s 78. He gets things wrong. Occasionally the white of his eye fills with blood. Pete Alonso he isn’t. The question is whether he’ll seem like Grandpa Simpson.

So the stakes are very high for him.

Or are they? If all he has to do tonight is not look like Grandpa Simpson, he might be home free. That’s not a very high bar to clear.