Candidates debate when they have to debate.

That’s why Donald Trump didn’t during the primaries.

That’s why Joe Biden, battling abysmal poll numbers, surprised everyone yesterday by agreeing to two debates.

And why Trump, who’s been demanding a side-by-side comparison – he had even floated debating on my show – immediately accepted.

BIDEN’S TEAM IS AFRAID TO ‘LET HIM LOOSE’: LUCY CALDWELL

With a speed that raised questions about whether it was "rigged," both camps agreed within a couple of hours to do the faceoffs on CNN on June 27, and on ABC Sept. 10. (Four networks were deemed eligible.) CNN has already announced Jake Tapper and Dana Bash as moderators.

While key details remain to be worked out, Trump appears to have agreed to two key Biden conditions. And this is not a man who likes others dictating the rules.

First, Biden doesn’t want an audience. He doesn’t want Trump, who draws huge crowds at rallies, whipping his supporters into hooting and hollering. (Trump said he wanted big venues for "excitement.") The president wants a quiet debate focused on policy. 

Second, Biden wants a mute button. That is, when one candidate is speaking for the allotted 60 or 30 seconds, the other’s mike is shut off. This is to prevent a replay of their first debate in 2020, when Trump was constantly interrupting or talking over Biden and moderator Chris Wallace. The former president can’t be thrilled about that.

CROSS-EXAMINATION THROWS MICHAEL COHEN OFF BALANCE, BUT BELABORS POINT THAT HE HATES TRUMP

This all went public at 8 a.m. Eastern yesterday morning, when Biden posted a short and snarky video:

"Now he’s acting like he wants to debate me again. Well, make my day, pal." 

By the way, the man doesn’t make a very convincing Clint Eastwood.

"I’ll even do it twice. So let’s pick the dates. Donald. I hear you’re free on Wednesdays." (The joke, in case you were wondering, is that the hush money trial isn’t in session on Wednesdays – like yesterday, when Biden’s message would actually get covered."

Trump responded on Truth Social that while "Crooked Joe Biden is the WORST debater I have ever faced" and "can’t put two sentences together," he accepts.

It’s hard to overstate the importance of these two events, which more than any debate since Ronald Reagan told Jimmy Carter in 1980 "there you go again," could decide the election.

Biden was on track to lose the election. He’s been shielded and hardly making any news, even before Trump’s hush money trial began. His team must belatedly recognize this. Joe had to do something to shake things up.

The Biden camp believes that his superior knowledge will become evident in no-frills debates. There’s also a conviction that the more the public sees of Trump, who’s been cooped up in a Manhattan courthouse, the better it is for the president.

On the other hand, Trump’s sheer physical presence, and bombastic style, will present a favorable contrast to Biden’s elderly mien and thin voice (though they’re only 3-½ years apart).

And trust me, as someone who’s sat down with Donald for an hour-long interview, he can focus and exercise discipline when he wants to.

SUBSCRIBE TO HOWIE'S MEDIA BUZZMETER PODCAST, A RIFF ON THE DAY'S HOTTEST STORIES

This probably sounds the death knell for the Commission on Presidential Debates, which has handled the fall events since 1988. It has become something of a dinosaur, locked into three October debates (after early voting starts) with a loud audience. Plus the moderators, picked from the usual suspects (except the late great Jim Lehrer) were often distrusted. So both candidates are bailing on the bipartisan panel.

Already there’s chatter that Trump might find reasons to back out, but I don’t see that happening. And I didn’t think his opponent would engage.

But Biden’s calculation is that he had to roll the dice. We’ll know soon enough whether his gamble pays off.