On Sunday evening, MSNBC’s Joe Scarborough, a near-relentless cheerleader for then-candidate Donald Trump during the primaries, explained his recent decision to leave the Republican Party: Donald Trump. He writes:

I did not leave the Republican Party. The Republican Party left its senses. The political movement that once stood athwart history resisting bloated government and military adventurism has been reduced to an amalgam of talk-radio resentments.

This, in and of itself, is a remarkable statement. It’s remarkable because Trump is the first Republican president since Dwight Eisenhower to embrace some form of quasi-isolationism; his entire campaign was run against the Iraq War. It’s also remarkable because perhaps Trump’s only major accomplishment aside from the nomination of Justice Gorsuch has been paring back the regulatory state. But what’s really irking Scarborough isn’t policy. It’s personality.

Scarborough didn’t leave the party when George W. Bush was president. He’s leaving now because Trump “questioned America’s constitutional system of checks and balances … echoed Stalin and Mao by calling the free press ‘the enemy of the people’ … insulted allies while embracing autocratic thug. ... It is a dying party that I can no longer defend.”

But Scarborough stood by Trump long into the primaries, while Trump was doing many of these things. He wrote columns defending Trumpism as a necessary backlash to Bush-era compassionate conservatism. His switch-in-time seems little more than an act of political convenience. Scarborough should know now that the takeover of the Republican Party isn’t all that difficult a proposition, given his own newfound scorn for Trump — but he isn’t sticking around to seek a corrective. Instead, he’s bolting.

The real reason? Because Scarborough may have ambitions of his own. He says that Trump will “lead to the election of independent thinkers no longer tethered to the tired dogmas of the polarized past.” This is what all politicians say — it’s what Barack Obama said repeatedly. So look for Scarborough to use his own irritation with Trump as a springboard for his future ambitions. But don’t assume it’s truly sincere. If it were, it would have driven him to oppose Trump long before Trump achieved political liftoff.