Political endorsements aren't what they used to be. Traditionally, when politicians would endorse a candidate, they'd defend that candidate from attacks. For Republicans getting behind Donald Trump, the opposite is happening.

Take House Speaker Paul Ryan. It was only last Thursday that he finally backed Trump in his hometown paper. Admittedly, it was a tepid endorsement. Ryan even promised to speak up when he has differences.

But now it doesn't seem like much of an endorsement at all. On Tuesday, Ryan said Trump's slur against a judge of Mexican descent was "the textbook definition of racism."

Other Trump endorsers have taken aim at the man they back for commander in chief. Sen. Bob Corker, R-Tenn., rumored to be on Trump's shortlist for vice president, said the nominee should acknowledge that he "stepped in it." Sen. Mark Kirk, who is in a tough re-election fight in Illinois, simply rescinded his endorsement, saying he could not support such a man.

The most dizzying series of reversals came from onetime candidate Marco Rubio, who had waged a spirited campaign against Trump — calling him a "political con man" — before the real estate mogul crushed him in Rubio's home state of Florida. Last month, Rubio said he would gladly speak for Trump at the party's convention this summer, saying he believed that Hillary Clinton would be an even worse president. This week, though, Rubio went back to his primary form. He said Trump's comments were "totally inappropriate" and declared that he wouldn't speak on his behalf at the convention.

Now, it should be said that Trump brings this on himself. Many Republicans had hoped that the presumptive nominee would exercise more self-control and act more presidential after wrapping up the nomination. But the man cannot help himself — hence the petty bigotry about Judge Gonzalo Curiel, who is presiding over a lawsuit against the now-defunct Trump University. As Bloomberg reported this week, Trump even overruled his campaign advisers, urging his supporters to continue attacking the federal judge.

At the same time, these thin endorsements prove a point that Trump has been making since he got in the race: The political class are a bunch of frauds.

Trump made this argument best at a town hall in New Hampshire last September. Surveying his competitors, Trump said that former Florida Gov. Jeb Bush — who was Rubio's mentor in Florida politics — had every reason to be furious that his disloyal protégée would run against him. And yet, Trump said, when the press asked both men what they thought of each other, they emphasized what great friends they were.

"They ask Bush, 'What do you think of Rubio?' [and he says] 'He's my dear friend, he's so wonderful, I love him so much,' " Trump said in a mocking tone. "They ask Rubio, 'What do you think of Bush?' 'Oh, he's my dear friend, wonderful.' "

Trump then said what everyone in the room already knew: "They hate each other. Trust me, I know. They hate so much, they hate more than anybody in this room hates their neighbor. But it's political bull----."

Now Rubio, Ryan, Corker, Kirk and many other Republicans are proving Trump's point about the political class. But this election cycle also has taken its toll on the presumptive Republican nominee. On the night that he wrapped up the nomination in Indiana, Trump graciously praised the last of his rivals, Texas Sen. Ted Cruz. Only a few hours earlier, he was telling reporters that Cruz's father may have had a role in the assassination of John F. Kennedy. But on victory night, Cruz was a "tough, smart guy" with a bright future in politics.

As Trump might observe, that was just "political bull----."