“These are all false accusations,” Mr. Trump said. “These are false. They’re trying to destroy a man.”
The White House did not immediately announce a nominee to replace Dr. Jackson. His withdrawal ensures that the department, which employs more than 370,000 people and includes vast health and benefits systems, will remain without a permanent leader for at least weeks to come.
The concerns raised on Capitol Hill over Dr. Jackson’s nomination were bipartisan and emerged after the Senate Veterans’ Affairs Committee interviewed more than 23 people, including current and former military personnel, who had worked alongside him. The accusations included a hostile work environment, the improper dispensing of prescription drugs to White House staff and reporters during official travel, and intoxication while traveling with the president.
The White House had initially moved to defend Dr. Jackson against what officials there called “ugly” abuse and false accusations. And he indicated repeatedly in interactions with reporters that he intended to stay the course.
But the nomination was clearly in peril when the top senators on the committee announced on Tuesday that they would postpone a confirmation hearing for Dr. Jackson scheduled for the next day, pending further investigation.
On Wednesday, the committee’s Democratic staff released a two-page document fleshing out the accusations. They were explosive.
In one instance, Dr. Jackson stood accused of providing such “a large supply” of Percocet, a prescription opioid, to a White House Military Office staff member that he threw his own medical staff “into a panic” when it could not account for the missing drugs, the document said.
In another case, at a Secret Service goodbye party, the doctor got intoxicated and “wrecked a government vehicle.”
And a nurse on his staff said that Dr. Jackson had written himself prescriptions, and when caught, had simply asked a physician assistant to provide him with the medication.
An aide to Senator Jon Tester of Montana, the top Democrat on the committee, said each of the allegations included in the document was based on information provided by two or more individuals.
President Trump nominated Dr. Jackson to the position in March after firing his first Veterans Affairs secretary, David J. Shulkin, an experienced hospital administrator and veteran of the department’s medical system. The decision was largely made out of a personal affinity for Dr. Jackson, who did not undergo the kind of policy vetting that usually accompanies a nomination to a cabinet post.
Mr. Trump had strongly defended Dr. Jackson on Tuesday as “one of the finest people that I have met,” but he also suggested that Dr. Jackson might soon withdraw from consideration, amid what the president characterized as partisan attacks from Capitol Hill.
“I don’t want to put a man through a process like this,” Mr. Trump said. “The fact is, I wouldn’t do it. What does he need it for?”
Sarah Huckabee Sanders, the White House press secretary, said on Wednesday that Dr. Jackson had been through at four background checks, including by the F.B.I., during his time at the White House. She said that none had turned up areas for concern.
But even before the accusations about his conduct became public, Dr. Jackson was expected to face tough questioning from senators from both parties skeptical of his inexperience managing a large bureaucracy and of his views on key policy debates gripping the department. The Veterans Affairs Department is the federal government’s second largest and Dr. Jackson had little to no experience with policy or leading a large staff.