WASHINGTON - When allegations of professional misconduct by White House physician Ronny Jackson started trickling during the past week to the Senate committee considering his nomination to lead the Department of Veterans Affairs, its chairman, Sen. Johnny Isakson, R-Ga., called the White House twice seeking information.

The answers did not appear to satisfy him.

The information void plunged Jackson's nomination into peril faster than any other in a series of controversial Cabinet choices that have marked the Trump presidency. It also prompted a new round of criticism of how President Donald Trump has been filling top administration posts - often without first conducting the kind of vetting that uncovers problems before they explode in public.

In Jackson's case, his nomination was already facing scrutiny from veterans groups and lawmakers who questioned his management experience, his views on outsourcing VA services and his glowing description of Trump's health during a January news briefing.

Then came charges that Jackson had contributed to a hostile work environment, had been seen drinking excessively on official trips and had improperly dispensed medications to White House staff members.

Jackson, who did not respond to requests for comment, denied any wrongdoing in meetings on Capitol Hill on Tuesday. And after Trump appeared to suggest Tuesday that Jackson should walk away from the job, White House officials indicated later that the administration planned to stand by the VA nominee.

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President Trump on Tuesday addressed the allegations involving Rear Adm. Ronny Jackson.

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The chaos triggered bewilderment on Capitol Hill, where some senators expressed concern about Jackson's hasty nomination process.

"Obviously, you want to make sure that when you get these nominees and their qualifications that a lot of this, the background, has been done," Sen. John Thune, R-S.D., said. "And it sounds like maybe in this case, it wasn't quite ready to send up here."

From the beginning, there were doubts about Jackson's readiness to lead the federal government's second-largest bureaucracy, an agency of more than 375,000 employees with a budget of more than $185 billion. The 50-year-old Navy rear admiral currently heads a staff of 70 at the White House, the largest team he has ever managed.

When Trump tapped Jackson for the post, the White House turned to mid-level staffers to brief him, some of whom lack deep knowledge of how VA works, according to people familiar with the situation. A junior press aide was tasked with escorting Jackson to Capitol Hill to meet with senators.

The process seemed disorganized, according to two VA officials with knowledge of how the preparations were handled. Jackson himself expressed frustration. At one point, he requested that the in-person briefings begin because "he said he was sitting by himself reading briefing books for days," one official said.

Meanwhile, over the past week, complaints about Jackson were quietly reaching senators on the Senate Veterans' Affairs Committee. Among the charges: that he improperly doled out prescription drugs and drank while on duty, Sen. Jon Tester, D-Mont., told NPR on Tuesday.

Sen. Sherrod Brown, D-Ohio, said his office had been contacted by numerous people, including veterans and active-duty military members, who voiced serious concerns about Jackson. "They're whistleblowers and they're not substantiated, but there's enough of them coming from credible people that you can't help but being concerned," he said.

Sen. Mike Rounds, R-S.D., another member of the committee, said his office received what he described as an "unsubstantiated allegation" against Jackson that his staff was investigating.

In all, more than 20 military employees spoke to senators about Jackson, Tester said.

"All I can tell you is we didn't initiate this discussion," Tester said. "This discussion came when we were notified by folks who work with Admiral Jackson, folks in the military."

Jackson denied any wrongdoing, according to Sen. Jerry Moran, R-Kan., who met with him Tuesday. In the meeting, Jackson also denied ever drinking on duty, Moran's spokesman said.

Several former colleagues described Jackson and Jeffrey Kuhlman, who served as personal physician to President Barack Obama until he retired in 2013, as rivals who regularly clashed with each other, contributing to a tense workplace.

Kuhlman declined to be interviewed for this article.

Two former colleagues of Jackson's, who spoke on the condition of anonymity to describe internal discussions, told The Washington Post that they believe he over-dispensed medications, including the sleep aid Ambien and the stimulant Provigil.

Both said the use of such drugs is common and necessary for the multi-national trips that dozens of White House aides must take with the president. But they said they thought that Jackson gave them out too frequently, especially for officials in positions of power with the ability to influence his career.

Others disagreed.

Brian McKeon, a former executive secretary and chief of staff of the National Security Council in the Obama administration, said in an interview that Jackson and other White House doctors would distribute Ambien freely if people requested it, but only in small doses, such as in an envelope with two or three pills.

McKeon - who saw Jackson frequently during presidential travel - said he never witnessed Jackson mistreating a member of his staff or being under the influence of alcohol.

"It just doesn't ring true, compared to my experience and interactions with him," McKeon said of the allegations. He said he told Tester's staff about his interactions with Jackson.

Jackson was already under unprecedented scrutiny for a VA secretary nominee - a post that in modern history has always won unanimous backing in the Senate.

Some of the tension stemmed from Trump's abrupt dismissal of his predecessor, David Shulkin, and Shulkin's public pushback against forces in the administration that he said were pushing him out.

His departure underscored how politicized the traditionally nonpartisan agency has become under Trump amid a polarizing debate over how much to outsource veterans' medical care to private doctors.

The issue has pitted almost every traditional veterans group against Trump loyalists and a Koch brothers-backed group that advocates for private care.

Jackson had lukewarm support from both factions, since his policy views were not known. But he told several senators in recent days that he opposes greater outsourcing of medical care to private doctors.

With little information available about the VA nominee, the charges against him quickly raised alarms on Capitol Hill.

The White House's failure to immediately address them prompted Isakson late Monday to indefinitely postpone Jackson's confirmation hearing, which was scheduled for this week.

The developments played out at such a rapid-fire pace that Jackson's fate appeared to ricochet in different directions Tuesday. In the morning, he was making the rounds on Capitol Hill, telling senators in private meetings that he had done nothing wrong and professing to reporters that he was looking forward to his hearing.

Less than an hour later, Trump was suggesting on live television that Jackson should think about withdrawing his nomination.

By the end of the day, Jackson was in the Oval Office, where Trump urged him to fight back.

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The Washington POst's Alice Crites, John Wagner, Erica Werner and Emily Wax-Thibodeaux contributed to this report.