Washington • President Donald Trump on Saturday called Sen. Mitt Romney a “pompous ass” after the senator criticized him for asking foreign powers to investigate a Democratic rival.

The expletive-laced tweet from the president was a warning, a shot across the bow that is likely meant to intimidate other Republicans. In short: Be loyal.

In a morning tweetstorm, the president lambasted Romney and said that he had begged for an endorsement to run for the Senate seat from Utah and previously to be secretary of state.

“Mitt Romney never knew how to win,” the president tweeted en route to his golf course in Sterling, Va. “He is a pompous 'ass' who has been fighting me from the beginning, except when he begged me for my endorsement for his Senate run (I gave it to him), and when he begged me to be Secretary of State (I didn’t give it to him).”

“He is so bad for R’s!” the president added, referencing Republicans.

Romney didn’t ask Trump for an endorsement for his Senate run in Utah, though Romney did accept it after the president tweeted his support. The Utah Republican met with Trump over dinner after the 2016 election when Trump was considering him to run the State Department. Trump instead chose former Exxon Mobil executive Rex Tillerson.

Facing an impeachment inquiry for his request that Ukraine investigate former Vice President Joe Biden and his son Hunter — as well as comments that China do the same — Trump has increasingly castigated Democrats for what he calls a witch hunt and “presidential harassment,” and lauded those who have gone on Fox News or otherwise defended him.

For those who haven’t had his back, Trump has leveled accusations of treason and let slip the words of war — actually at one point tweeting that if he’s removed from office, it would spark a “Civil War-like fracture.”

The tweet about Romney was a message, says John Weaver, a longtime GOP consultant who has criticized Trump.

“Like any bully — or Queens crime boss — he’s trying to make an example of someone who steps out of line,” Weaver said Saturday of the New York-born president. “The only thing keeping him from being ousted is the combination of careerism and cowardice in the GOP Senate conference, so keeping them cowed is his main focus.

“But he has no leverage over Mitt Romney," Weaver added, “and that must frighten him.”

Romney is one of the few Republicans who has been willing to raise concerns about Trump’s comments and actions. Most GOP members of Congress have either remained silent or blamed Democrats for targeting the president, including Utah Rep. Chris Stewart, who has appeared several times this week on Fox News to defend Trump, and Sen. Mike Lee, who has said he sees no problems with the president’s actions.

Romney, who declined to comment Saturday, has never been a Trump supporter, calling him out as a “phony, a fraud” during the Republican primaries and offering occasional criticism of his actions and remarks. Romney voted for his wife, Ann, rather than Trump in 2016.

As the news emerged that Trump had asked Ukraine President Volodymyr Zelensky to investigate the Bidens, and then told reporters that China should as well, Romney spoke out.

“When the only American citizen President Trump singles out for China’s investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated,” Romney wrote Friday on Twitter.

Trump supporters have targeted Romney for being a vocal critic. Conservative commentator Rush Limbaugh told his massive Trump-loving audience last week — falsely — that Romney had told House Speaker Nancy Pelosi, D-Calif., that there were enough votes to support removing Trump from office. Romney and Pelosi haven’t talked since February.

The president, who has previously ridiculed Romney for losing the 2012 presidential race, responded Saturday that there was nothing wrong with a phone call with the Ukrainian president.

“Somebody please wake up Mitt Romney and tell him that my conversation with the Ukrainian President was a congenial and very appropriate one, and my statement on China pertained to corruption, not politics. If Mitt worked this hard on Obama, he could have won. Sadly, he choked!” Trump tweeted.

Later, after returning to the White House from the visit to his golf course, Trump again went after Romney, saying that the Utah senator is worse than former Sen. Jeff Flake, R-Ariz., who had been a sharp Trump critic.

The president added:

“I’m hearing that the Great People of Utah are considering their vote for their Pompous Senator, Mitt Romney, to be a big mistake. I agree! He is a fool who is playing right into the hands of the Do Nothing Democrats!”

Romney won 63% of the vote in Utah in 2018. Trump took Utah in the 2016 election but by less than a majority — 45.5% of voters cast ballots for him — against Democrat Hillary Clinton and independent Evan McMullin.

When President Bill Clinton faced impeachment in the late 1990s, his White House set up a war room to combat Republicans and keep Democrats in check. Trump’s White House hasn’t followed suit, instead letting the president command his own responses, which have often come in the form of tweets or off-the-cuff remarks when reporters are present.

Doug Heye, a former head of communications at the Republican National Committee and a CNN contributor, said Saturday that Trump's attack on Romney was likely a move to keep Republicans in line.

“The White House is very mindful that they are in such a delicate situation, such a tenuous position that they cannot afford cracks in the dam,” Heye said. “And, you know, one senator is one thing but if one senator becomes three senators and two members of Congress, then the whole dam could burst pretty quickly. And so they know that they have to, just for their own sake of survival, push back very strongly. Unfortunately with the president, that means he’s going to be personal and ugly very quickly.”

Rick Klein, political director of ABC News, tweeted Saturday that Romney has essentially become the de facto compass for the Republican Party during a time when GOP members are unsure how to respond to the president.

“People can mock Mitt Romney,” Klein wrote. “But he’s using his platform as a United States senator at this moment. A lot of Republicans are looking to him for leadership at this moment.”

But few, so far, have been willing to put themselves in the president’s crosshairs.