President Donald Trump escalated his feud with Democratic Sen. Jon Tester on Saturday morning, using a tweet to call for the Veterans Affairs Committee ranking member to resign.

The Montana senator on Wednesday made public allegations from whistleblowers against Navy Rear Adm. Ronny Jackson, Trump’s military physician and nominee to run the Department of Veterans Affairs, that helped him decide to step aside.

Jackson is one of many Trump nominees or appointments to run into trouble over job qualifications or allegations of misconduct, including former VA director David Shulkin, who left in March after a troubled tenure; former Health Secretary Tom Price, who resigned in September under a cloud; and former White House national security adviser Michael Flynn, who pleaded guilty late last year to criminal charges. And staff turnover in the Trump White House has been unprecedented.

Tester and Chairman Johnny Isakson, R-Ga. had released a statement voicing concerns about Jackson.

“We take very seriously our constitutional duty to thoroughly and carefully vet each nominee sent to the Senate for confirmation,” the joint statement read. “We will continue looking into these serious allegations and have requested additional information from the White House to enable the committee to conduct a full review.”

Jackson, who also cared for former presidents George W. Bush and Barack Obama, has denied accusations that he was intoxicated during working hours, crashed a government vehicle, handed our prescription drugs too freely and fostered a hostile work environment.

Trump continued defending Jackson on Saturday, tweeting his contention that “Allegations made by Senator Jon Tester against Admiral/Doctor Ron Jackson are proving false.”

Trump pointed to a U.S. Secret Service statement on Thursday that said the agency has been unable to find any records of an incident involving a drunk Jackson and a wrecked government automobile.

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“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,” the president wrote.

Then he issued his challenge, writing this: “Tester should resign.”

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

Sen. Jon Tester replied to Trump’s call for his resignation by saying in a statement it is “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.”

His office noted in the same statement that the president “has signed eight of Tester’s bills into law to hold the VA accountable and improve care and benefits for veterans.”

The allegations against Jackson are still of interest to the Senate because his nomination to receive a second star is still pending before the Armed Services Committee. An aide to ranking member Sen. Jack Reed, D-R.I., told Roll Call the Pentagon will need to review the allegations before any action is taken on the military promotion.

Trump and GOP officials quickly pounced on Tester earlier this week, eager to keep the Senate in their control. Tester faces a tough re-election fight in November’s midterm elections, with Inside Elections with Nathan Gonzales/Roll Call ratings putting the Senate race there as Tilt Democratic.

“The Secret Service has no such record of any incident; specifically, any incident involving Rear Admiral Ronny Jackson,” the agency said in that statement. “A thorough review of internal documents related to all Presidential foreign travel that occurred in 2015, in addition to interviews of personnel who were present during foreign travel that occurred during the same timeframe, has resulted in no information that would indicate the allegation is accurate.”

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On Friday, the National Republican Senate Committee also seized on the Secret Service statement to go after the vulnerable Tester as they try to claim his seat. The Senate GOP’s campaign arm issued its own statement that featured this headline: “Tester’s gamble backfires.” The Senate GOP’s campaign arm echoed recent statements from the president, saying Tester’s actions are “sure to hurt his re-election prospects in a state Trump won by more than 20 points.”

The GOP president on Saturday continued his apparent use of the matter to try to fire up the Republican base in Montana, writing in a second tweet that the “great people of Montana will not stand for this kind of slander when talking of a great human being.

He then, like during a joint press conference Friday with German Chancellor Angela Merkel, heaped praise on his military doctor.

“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,” Trump wrote. “Not fair, Tester!”

….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

During a roller-coaster-like phone interview on Thursday with “Fox & Friends,” his favorite morning news program, Trump said the Veterans Affairs ranking Democrats should have a “big price to pay” over the Jackson matter.

On Tuesday, Trump acknowledged he created an “experience problem, because lack of experience,” a reference to Jackson’s lack of major command or management experience. GOP and Democratic lawmakers, soon after the spur-of-the-moment nomination was announced by Trump on Twitter, questioned whether the military doctor was qualified to run a sprawling federal agency that is plagued by myriad problems.

Watch: Trump Stands Behind VA Pick, But Says ‘I Wouldn’t Do It’