WASHINGTON — White House officials on Tuesday defended Veterans Affairs secretary nominee Ronny Jackson as Senate officials signaled serious concerns about whether to ever allow him a confirmation hearing for the post.

Jackson, whose surprise nomination has been shrouded in questions about his lack of experience with the massive veterans bureaucracy, was scheduled to appear before the Senate Veterans’ Affairs Committee on Wednesday before an abrupt postponement on Monday night.

In a letter to the White House Tuesday morning, committee chairman Johnny Isakson, R-Ga., and ranking member Jon Tester, D-Mont., requested the administration turn over all documents related to Jackson’s work at the White House medical office and during his Navy career, including allegations of misconduct and poor leadership against him.

In a statement to the press, the two senators said they had postponed the hearing “in light of new information presented to the committee.”

They declined to specify what the allegations are against Jackson, a 23-year naval officer who deployed to Iraq as a combat surgeon.

“We take very seriously our constitutional duty to thoroughly and carefully vet each nominee sent to the Senate for confirmation,” the senators said. “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.”

Rumors of concerns about Jackson’s handling of pain medications have been circulating on Capitol Hill for the last week, since the White House formally submitted Jackson’s nomination to the Senate. In addition, other news reports have raised questions of improper alcohol use and lack of professionalism in the White House medical office under Jackson’s leadership there.

But in a statement Tuesday morning, White House officials said they stand by Jackson.

“Admiral Jackson has been on the front lines of deadly combat and saved the lives of many others in service to this country,” Deputy White House Press Secretary Hogan Gidley said in a statement.

“He’s served as the physician to three presidents — Republican and Democrat — and been praised by them all. Admiral Jackson’s record of strong, decisive leadership is exactly what’s needed at the VA to ensure our veterans receive the benefits they deserve.”

Jackson, a Navy rear admiral and who currently serves as the White House physician, was tapped for the VA post last month after President Donald Trump fired then-Secretary David Shulkin over Twitter amid ethics concerns and department infighting.

But Jackson’s lack of experience with VA — or managing any other large health care system — drew immediate concerns from veterans groups and lawmakers.

Individuals involved in the process also raised questions about how thoroughly Jackson was vetted before being asked to take over the 375,000-employee VA bureaucracy. In the weeks before Shulkin’s firing, Jackson was interviewed as a potential VA under secretary for health but rejected because the White House pulled him back.

After the committee announcement, Senate Minority Leader Chuck Schumer, D-N.Y., took to the Senate floor to blast the Trump administration for it’s “poor record” of handling nominees over the last year, and questioned the “troubling allegations” against Jackson.

“How did he get through the process with all of these allegations, not even being made public?” Schumer said. “My guess, not proper vetting. I wasn’t there, but it’s speculative that maybe one day that the president, who we know acts on impulse, had this nominee in the room, his doctor, and said, ‘Hey, let’s put you up without any vetting.’”

Senators did not announce a new date for Jackson’s confirmation hearing, raising doubts about whether one will happen at all.

Shulkin’s firing was the second time in less than four years the department lost its leader in disgrace, and the Trump administration for the last year has had difficulty filling a host of high-profile VA jobs. Now, with Jackson’s confirmation delay, the department faces an uncertain timetable for when its critical leadership posts will be filled.

Trump appointed Defense Department Under Secretary for Personnel and Readiness Robert Wilkie to take over as acting secretary after Shulkin’s dismissal, with an eye toward him filling that role only for a few weeks or months. In doing so, they bypassed Deputy VA Secretary Thomas Bowman, a move that several veterans groups insist may have violated federal law.