Republican congressmen and senators met with Donald Trump on Thursday. It did not go well.

Trump reportedly insulted Sen. Mark Kirk (R-Ill.), calling him a loser. Trump then predicted Sen. Jeff Flake — an enormously popular Republican from Arizona — would lose re-election this year, even though he isn't up for re-election. This was after Flake had said he wanted to find a way to support Trump and get over his concerns. Rep. Mark Sanford (R-S.C.) left the meeting questioning Trump's basic grasp of the Constitution.

This was on the heels of a report in The New York Times in which Trump was asked if he would serve as president if elected. "I'll let you know how I feel about it after it happens," Trump replied, flashing what the Times deemed a "mischievous smile."

This was also a week in which Trump defended his campaign's decision to source anti-Semitic clipart from a fan's Twitter account, titillated the media with pro-Saddam soundbites, and gave a manic, rambling, stream-of-consciousness speech in Cincinnati in which he spent a great deal of time talking about a golf course in Scotland.

This should have been a great week for Trump. This was the week the director of the FBI ascended the stage of our politics to denounce Hillary Clinton's recklessness with state secrets and security on her home-brew email servers. But Trump seemed to prefer the spotlight be on him, even if it meant dropping a curtain in front of FBI director James Comey and doing a rake-stepping vaudeville show in which, for his macabre finale, he truly hung himself.

In other words: Donald Trump is not going to get out of Donald Trump's way to the presidency. At this point, Sen. Flake and other holdouts, like Sen. Ben Sasse (R-Neb.), should know that there is no conceivable upside to endorsing Trump. The same goes for Mitt Romney, George H.W. Bush, George W. Bush, and every single GOP governor and Republican in the House and Senate who hasn't endorsed Trump. This column is for them.

Don't endorse Trump. You have nothing to gain, and everything to lose.

Even if you strip away every political consideration besides the desire to toady up to Trump, the fact is this: If you have held out on endorsing this long, you are already on Trump's naughty list. Plus, endorsing now signals to Trump your servility, and to the public, cravenness. Trump is already offering convention speaking slots to the likes of Don King and Bobby Knight. A mid-July endorsement is not going to win his favor.

And really: Trump has done nothing since earning the "presumptive" tag to merit an endorsement — unless you think attacking a judge for his ethnicity and praising dictators counts.

Even if Trump could convince a holdout to come over now, what is the best case scenario? A place in the Cabinet? That requires Trump winning the election, which is the worst case scenario for Republicans. It would mean enduring four more years of him destroying the Republican brand. To anyone under 40, the only Republican presidencies clearly remembered would be those of George W. Bush and Donald Trump. After inducing that kind of national trauma, the Republican Party would be lucky to have even legal sanction for continued existence, let alone elected officials.

So far, it seems, no Republican who has remained silent has experienced anything more troubling than light rebukes from RNC Chairman Reince Priebus. These should be taken for what they're worth, a good laugh.

The downsides to endorsing Trump are, of course, tremendous. Trump has shown no grip over his passions, or on basic facts about the structure of the United States government. He will not "pivot" or "evolve." His positions will certainly change with the exigencies as Trump understands them in his addled mind. But that is only the deeper continuity of Trump's lack of principle.

Endorsing Trump means that reporters and political opponents get to ask you whether you still support him after his next outrage. If Trump praises Kim Jong Un for his toughness in murdering his own family members, what will your answer be?

Trump is a man who has engaged in the lowest forms of exploitative buck-raking, including but hardly limited to Trump University. He is unfaithful to every view and promise he makes, to wives or creditors. If Trump's mischievous smile to The New York Times on his willingness to serve is an indication that he has been engaged in an elaborate prank on the GOP, you can't say that you didn't see the signs coming.

Do you really want to be one of the last men or women in on the joke?