Joe Scarborough knows how to push Donald Trump's buttons. Now, he's sharing the secret.

Seemingly chummy early in the campaign, the MSNBC host and the presumptive Republican presidential nominee aren't so friendly these days. Scarborough has ripped Trump's attacks on U.S. District Court Judge Gonzalo Curiel as "completely racist," and Trump has said he won't appear on "Morning Joe" — where he used to be a frequent phone-in guest — anymore.

But condemning Trump's rhetoric isn't the way to get under his skin, according to Scarborough. In a podcast interview posted online Thursday, the former Republican congressman told Politico's Glenn Thrush what really bothers Trump:

Ratings. Numbers. Saying that Bernie Sanders draws more people than him. Mocking him for only having 5,000 people at [a Washington, D.C., rally over Memorial Day weekend] when he thought he was going to have 400,000. ...
I made the mistake, in Donald Trump's eyes, of saying that actually Chris Christie … would out-rate Donald Trump. Bernie Sanders would out-rate Donald Trump [on "Morning Joe"]. You want to get to Donald Trump? That's how you do it. I got a long letter [from Trump], talking about spreadsheets and how he actually out-rated Chris Christie. … We just kind of checked. How did Trump do yesterday? [Do] people flock when Donald Trump came on? Maybe they still do in prime time; I don't know. But they didn't, at least for our show.

Scarborough summarized Trump's weakness in a single adjective: "insecure."

We should note that Scarborough, as a conservative commentator, plays a different role from the neutral one many journalists try to fill. In general, it isn't the media's job to get in Trump's head and exploit his supposed insecurities.

But Scarborough's insight does have some application for the news media at large. It's a reminder of the importance of fact-checking Trump's many grand claims — about his wealth, his crowds, his business acumen and, yes, his media magnetism.

Coverage of his provocative statements and policy prescriptions remains critical, of course, but so much of Trump's campaign is rooted in his own image of greatness that it is also important to investigate just how great he actually is.

If journalists keep doing that — and keep turning up evidence of exaggerations — the results will, according to Scarborough, be much more likely to elicit a response from Trump.