Sen. Jon Tester (D-MT) has become the target of President Donald Trump’s wrath after leading the charge in taking down the president’s pick to lead the department of Veterans Affairs, White House physician Ronny Jackson. Trump attacked the Montana Democrat on Twitter on Saturday morning, calling for his resignation, and later in the day called him “very dishonest and sick.”

Tester, who is the ranking member of the Veterans’ Affairs Committee, raised concerns about Jackson after multiple allegations surfaced that he oversaw a “toxic” work environment, overprescribed medication, and drank while on the job as a top White House doctor. Jackson withdrew his nomination on Thursday after those allegations were publicized in a memo released by committee Democrats.

Tester and Democrats weren’t the only ones to sound the alarm about Jackson. Many Republicans in the Senate, including Veterans’ Affairs Committee Chairman Johnny Isakson (R-GA), also voiced concerns. Isakson and Tester on Tuesday announced their decision to postpone Jackson’s confirmation hearing and wrote a letter to the president asking for information. But Trump has singled Tester out for attack.

In a freewheeling interview with Fox & Friends on Thursday, the president said Tester would have a “big price to pay in Montana” after his actions. At a press conference with German Chancellor Angela Merkel on Friday, Trump said what Tester had done was an “absolute disgrace.”

And on Saturday morning, Trump continued his offensive on Twitter, complaining that Jackson’s reputation had been “shattered” because of Tester and that the allegations against him are “proving false.” He called for Tester to step down. The senator is up for reelection in Montana in 2018.

Allegations made by Senator Jon Tester against Admiral/Doctor Ron Jackson are proving false. The Secret Service is unable to confirm (in fact they deny) any of the phony Democrat charges which have absolutely devastated the wonderful Jackson family. Tester should resign. The..... — Donald J. Trump (@realDonaldTrump) April 28, 2018

....great people of Montana will not stand for this kind of slander when talking of a great human being. Admiral Jackson is the kind of man that those in Montana would most respect and admire, and now, for no reason whatsoever, his reputation has been shattered. Not fair, Tester! — Donald J. Trump (@realDonaldTrump) April 28, 2018

Trump later in the day attacked Tester again, saying the Secret Service had told him Tester’s statements on Jackson “are not true” and comparing the situation to the ongoing Russia investigation, which he called “phony.” He said Tester “should lose” the race in Montana.

Secret Service has just informed me that Senator Jon Tester’s statements on Admiral Jackson are not true. There were no such findings. A horrible thing that we in D.C. must live with, just like phony Russian Collusion. Tester should lose race in Montana. Very dishonest and sick! — Donald J. Trump (@realDonaldTrump) April 28, 2018

Tester responded to Trump’s criticisms in a statement on Saturday. “It’s my duty to make sure Montana veterans get what they need and have earned, and I’ll never stop fighting for them as their Senator,” he said.​​

Jackson already withdrew his nomination, but the White House isn’t ready to let it go.

The White House has defended Jackson and disputed many of the allegations made in a memo distributed by Veterans’ Affairs Committee Democrats this week about him. The two-page document, first published by the New York Times, listed a number of troubling allegations — including that Jackson wrote himself prescriptions before getting caught, provided a “large supply” of the drug Percocet to a White House military aide without filling out the proper paperwork, and crashed a government vehicle while drunk.

The Washington Post reported on Friday that White House officials had conducted a review of Jackson’s vehicle records and found three minor incidents but no evidence that he “wrecked” a car. The memo said Jackson had crashed a government vehicle during a Secret Service going away party while intoxicated. Per the Post, the White House also put out two years of audits of the White House Medical Unit’s handling of prescriptions and medications, which showed no problems.

On Thursday, the Secret Service released a statement disputing another allegation that Jackson had banged so loudly on a female colleague’s door while drunk that the agents stepped in to stop him so he wouldn’t wake then-President Barack Obama during a 2015 overseas trip. The Secret Service said it has “no such record” of the incident.

Jackson denied all the allegations against him in his statement announcing his decision to withdraw from the VA secretary nomination, calling them “completely false and fabricated.” He said that if the allegations were true, “I would not have been selected, promoted and entrusted to serve in such a sensitive and important role as physician to three presidents over the past 12 years.” He served as a White House physician under Presidents George W. Bush, Obama, and Trump.

Deputy White House Press Secretary Raj Shah in a statement to the Post took aim at Tester. “Sen. Jon Tester engaged in character assassination against a decorated rear admiral in the United States Navy, and he didn’t have a shred of evidence to back it up,” he said.

Tester has stood by his decision to release the allegations against Jackson. “Look, there was information, there was a pattern to the information,” he said in an interview with the Post on Capitol Hill on Thursday. “I thought it was the right thing.”

Sen. Isakson is backing Tester up. A spokeswoman for him told CNN’s Jeff Zeleny that he has no problem with how things were handled and that she doubts he’d seen the president’s tweets.

In response, a spokeswoman for @JohnnyIsakson, GOP chair of VA committee: “Senator Isakson has a great relationship with Senator Tester. He doesn’t have a problem with how things were handled. I don’t know for sure but highly doubt he’s seen the president’s tweets this morning.” https://t.co/8AGjhmLNdb — Jeff Zeleny (@jeffzeleny) April 28, 2018

Regardless of the allegations against him, Jackson was not qualified to run the VA.

Questions about Jackson’s qualifications to run the VA came up almost as soon as Trump tweeted his intention to nominate him to replace former VA Secretary David Shulkin on March 28. As the White House’s top doctor, Jackson manages a staff of about 70; the VA employs 375,000 people and has a budget of more than $185 billion.

Vox’s Matt Yglesias recently outlined the case against Jackson, noting that allegations aside, he doesn’t have the experience to run one of the federal government’s most sprawling agencies:

Jackson was a successful combat surgeon who oversaw a few dozen staffers at the peak of his wartime service and now supervises a small staff of about 70 people in the White House medical office. He appears to struggle with the (modest) management aspects of the job but has succeeded nonetheless due to his strong personal rapport with a series of presidents. Trump likes him, in particular, because of his virtuoso press conference defending Trump’s health (including a height and weight estimate that registered him as officially non-obese), but this has nothing to do with the VA’s work. Even if the charges against Jackson are entirely false, “wasn’t drunk on the job” and “the president likes him” is not an appropriate bar for an important substantive position that requires considerable management skill and subject matter expertise. On some level, there’s no need to vet Jackson or look into these charges in a serious way because the idea of installing him should be rejected out of hand.

It’s now back to the drawing board for Trump on finding someone to head the VA. He told Fox & Friends on Thursday that he wants somebody “great” but lamented how hard the agency is to run for anybody. “Nobody has experience, you know?” he said. “It’s a big monster.”