To understand why Donald Trump took a shot at Anderson Cooper in an interview with The Washington Post on Thursday, you have to go back to something he said three days earlier. Reflecting on last week's Commander-in-Chief Forum, moderated by NBC's Matt Lauer, Trump said Monday on CNBC that "everyone's saying that [Lauer] was soft on Trump" — which is pretty much true.

Trump then explained what he thinks criticism of Lauer means for the upcoming presidential debates: "Now the new person is going to be really hard on Trump just to show the establishment what he can do."

Clearly the Republican nominee is worried about the political equivalent of a make-up call in sports. He knows many journalists believe Lauer blew the call, so to speak, by failing to whistle Trump for claiming falsely that he opposed the invasion of Iraq. And he thinks Cooper and the other debate moderators — Lester Holt, Martha Raddatz and Chris Wallace — will overcompensate by being extra tough.

Trump wants to prevent that from happening. So he's working the refs.

Thus we have these remarks to The Post's Robert Costa in Ohio: "I don't think Anderson Cooper should be a moderator because Anderson Cooper works for CNN, and over the last couple of days, I've seen how Anderson Cooper behaves. He'll be very biased, very biased. I don't think he should be a moderator. I'll participate, but I don't think he should be a moderator. CNN is the Clinton News Network, and Anderson Cooper, I don't think he can be fair."

These are the people who will be lobbing questions at Republican Donald Trump and Democrat Hillary Clinton when they go head-to-head at the 2016 presidential debates. The first debate was September 26. (Sarah Parnass/The Washington Post)

Recall that two days before the forum with Lauer, Trump praised the upcoming debate moderators. "I like them," he told reporters aboard his plane last Monday. "I respect the moderators. I do respect them."

During the Republican primary, Trump participated in two town hall events moderated by Cooper and did not object to the host's participation or complain about his performance. In a CNN interview after a GOP debate in March, Trump offered unprompted congratulations to Cooper for his moderation of a Democratic debate four days earlier.

"Anderson got very good ratings," Trump said. "We're very happy for Anderson. I don't know if it helps me, but he certainly got very good ratings."

It appears that Trump doesn't really have a problem with Cooper. But he suggested to Costa, in his oblique way, that something changed "over the last couple of days," something about "how Anderson Cooper behaves."

What is he talking about? The most logical guess would seem to be the interview Cooper conducted with Hillary Clinton on Monday evening. It was Clinton's first interview since she fell ill at a 9/11 memorial service on Sunday and revealed that she has pneumonia.

"There's a lot of folks who are very worried about you," Cooper began. "How are you feeling?"

Cooper was just being human, of course — even Trump has been wishing Clinton a speedy recovery this week — but Trump is suggesting that Cooper is too cozy with the Democratic nominee by saying "he'll be very biased" and attaching him to the "Clinton News Network."

Trump's previous statements suggest he doesn't actually believe Cooper will be unfair. He is just trying to soften the "AC360" host ahead of a debate.

The veteran CBS reporter, who moderated three presidential debates, looks back at key moments from those clashes and gives advice to his 2016 counterparts. (Adriana Usero,Erin Patrick O'Connor/The Washington Post)