US President Donald Trump has called on a Democratic Senator to resign over his opposition to White House physician Ronny Jackson's nomination for Veterans Affairs (VA) secretary, the media reported. Montana's Jon Tester, the top Democrat on the Senate Veterans Affairs Committee, had raised concerns about allegations against Jackson, including that he loosely handled prescription pain medications, was intoxicated during an overseas trip and created a toxic work environment, reports CNN. Jackson withdrew his nomination last week. In a series of tweets on Saturday, Trump said: "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 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!" Jackson initially did not respond to the full slate of allegations, but on April 26 when he pulled out as VA secretary nominee, he said: "The allegations against me are completely false and fabricated. If they had any merit, 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." In response to Trump's tweets, Tester said: "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."