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(CNN) —  

Republicans and Democrats on the Senate Veterans Affairs Committee are raising concerns about allegations involving Rear Adm. Ronny Jackson, President Donald Trump’s nominee to lead the Department of Veterans Affairs, and are reviewing them to determine if they are substantial enough to upend his nomination.

Committee members have been told about allegations related to improper conduct in various stages of his career, two sources said.

The sources say the committee is in talks to delay Wednesday’s confirmation hearing as they try to figure out the allegations.

None of the senators would publicly detail the specifics of the allegations. A White House spokesman did not immediately respond to a request for comment.

Two Republican sources with direct knowledge of the situation say the White House is aware that both Democratic and Republican lawmakers are looking into potential problems. Republican lawmakers and aides on the committee say they are also grappling with the allegations, trying to assess what they mean for Jackson’s nomination to the post.

Senate Democrats huddled privately Monday in a basement office in the Capitol to discuss “raw allegations” that have been raised against Jackson.

“There are reasons, as there are with every presidential appointee, for very close scrutiny and vetting. We need to know if allegations raised by others may have some factual basis. That’s the process of vetting that has to occur,” said Sen. Richard Blumenthal, D-Connecticut, as he left the meeting, which lasted about a half hour.

“We need to be asking questions and there need to be answers,” he added. “At this point we are dealing with some fairly raw allegations and we need to know if there is factual support for it.”

“All I can really tell you at this moment time is we are continuing the vetting process. We are working very hard at it. It’s all hands on deck,” said Sen. Jon Tester of Montana, the top Democrat on the committee.

Asked if the allegations were troubling, Tester replied, “Only if true.” Asked if Democrats could determine if the allegations are true before Wednesday’s hearing or if he would need to meet with Jackson again, Tester responded, ‘We’ll see.”

“I’m going to let my staff and myself enough time to do as much work as we can in the next 48 hours, less than 48 now. And then we’re gonna see what we can come up with and go from there,” Tester said.

Jackson was named to succeed David Shulkin at the Department of Veterans Affairs after Shulkin was ousted last month. Shulkin faced ethical issues and tensions with political appointees within the agency and the White House. A White House official told CNN that Trump was pleased with Jackson’s performance in an extended grilling by reporters in January over the President’s health and cognitive fitness.

But Jackson’s selection was met with questions on Capitol Hill, where lawmakers in both parties have called into question whether Jackson has the right experience to run the federal government’s second-largest bureaucracy and that little has been known, until recently, about his policy views.

“I’m not sure anyone can run the VA,” Senate Majority Whip John Cornyn told reporters Monday. “It’s so big, it’s one of the biggest bureaucracies in the federal government.”

The White House is closely monitoring Jackson’s confirmation process, and aides have been working for weeks to prepare him for his confirmation hearing. Mock hearings were held throughout the weekend, aides said, trying to prepare Jackson for Senate questioning.

One administration official said Monday the President had faith in his nominee and has no reason to believe Jackson’s confirmation is in jeopardy.