On Saturday, President Donald Trump took to Twitter to blast Montana Democratic Sen. Jon Tester (D-Montana) calling him “very dishonest and sick,” over his opposition of 12-year White House physician Ronny Jackson’s nomination for secretary of veterans affairs due to allegations that arose against him.

In a speech on Saturday night, Trump said he had “dirt” on Tester, but refused to elaborate.

Since the president first nominated Jackson last month, both Democrats and Republicans have expressed their skepticism in the White House physician’s ability to lead the U.S. Department of Veterans Affairs.

After nearly two dozen complaints from current and former colleagues in the weeks that followed, Tester, the top Democrat on the Senate Veterans Affairs Committee expressed his own concerns about handing the reigns of the second largest government agency over to a man who’s accused of making liberal use of prescription pain medications and crashing a car while intoxicated at a White House function, something Jackson denies.

In a series of tweets, Trump defended Jackson saying that “allegations made by Senator Jon Tester against Admiral/Doctor Ron Jackson are proving false,” and that “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.”

On Thursday, Jackson officially withdrew his name from consideration for the position. At the White House on Wednesday, Jackson denied having wrecked a government car saying “I have no idea where that is coming from.” 

Tester is locked in a tough re-election campaign in a state that went overwhelmingly for Trump in 2016.