(CNN) Republican lawmakers and aides have grown increasingly frustrated with the White House over the last several days -- tied in part to the unconventional nomination of Dr. Ronny Jackson to lead the Department of Veterans Affairs and the lack of congressional consultation before it was made.

"I understand the President wants his people and we want to be deferential as much as we can, but it would be nice to know some of the issues that come up after the fact, before the fact," said John Thune of South Dakota, the Senate's No. 3 Republican.

Lawmakers said they have received information that alleges Jackson over-prescribed sleeping medication and wakefulness medication during his tenure, as well as claims that an alleged power struggle with another White House physician decimated morale under his leadership in the White House Medical Unit.

Jackson also faces allegations that he drank on the job -- claims some White House sources characterized as false.

The White House doctor is just the latest in a string of nominees -- including Gina Haspel, whom Trump tapped to lead the CIA -- facing resistance unexpected by the White House. Repeatedly, the Trump administration has failed to recognize potential pitfalls for their nominees or worked to ensure Republican lawmakers in charge on Capitol Hill are on board.

A senior White House official told CNN on Tuesday that some aides believe Jackson is being "railroaded" by false claims designed to sink his nomination to replace David Shulkin, the VA secretary Trump ousted last month. Multiple sources said Jackson has no immediate plans to withdraw his name from consideration, but all said Jackson will make the final decision about whether to walk away from the process

"He's in it to win it. All up to him," said one White House aide.

On Tuesday, Trump conceded his doctor's nomination faces major problems.

"He is a high-quality person. It's totally his decision. So he'll be making a decision," Trump said.

White House press secretary Sarah Sanders defended Jackson on Wednesday morning, calling him a "very highly qualified, highly respected person in the military and the medical community."

"I'm not going to go line-by-line on every outrageous thing out there right now, but he certainly discussed them," she said of Tuesday's Oval Office meeting between Jackson and Trump.

Increasingly frustrated

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For days, Republican aides described several senators who reached out to the West Wing for answers to the allegations only receive little, if any, information about the veracity, severity or whether the White House was even aware of the extent of them.

White House talking points defending Jackson only landed in Senate Republican inboxes Tuesday evening, multiple aides said -- and weren't markedly different than what had been sent to the press.

That has only served to further sap the willingness among GOP lawmakers to go to bat for a nominee who was already lacking natural allies on the Hill.

"We're willing to keep our powder dry," one senior aide said. "But you aren't going to see many of our guys go out of their way to defend him until we get answers."

For the moment, Republican senators declining to attack Jackson or call for his nomination to be pulled is the best the administration can likely hope for, one senior GOP aide said.

"We're all in wait-and-see mode right now," the aide said. "We'll see how long that lasts."

The White House, meanwhile, is frustrated by the stream of critical headlines and interviews about Jackson, two officials said, but conceded they are struggling to turn the tide in their quest to defend him.

The officials said the West Wing has had little success reaching out to Republicans, in hopes of finding senators or other surrogates to push back against the narrative forming against Jackson.

Some White House officials said they were aware of some of the allegations against Jackson, but no one seemed to suggest they knew about all of them, as sources characterized some of the allegations as false. But Tuesday's postponement of Jackson's Senate confirmation hearing left White House staffers scrambling for answers to figure out what was going on.

During Jackson's review process with the White House's Presidential Personnel Office, the biggest concern flagged to some White House officials was his lack of experience -- not the other allegations which have surfaced.

Hurried decision

Among Jackson's allies, even within the West Wing, some have grumbled that Trump's hurried decision to elevate him without extensive vetting has essentially let Jackson hang out to dry.

"He got totally hosed," said one current White House aide, who said that a more thorough look at Jackson's background before his nomination might have helped mitigate his problems.

Others who have worked with Jackson, and remain supportive of the well-liked White House doctor, say his reputation and career could now be at stake -- an outcome that might have been prevented if he was properly vetted.

"What can he do now?" one former senior administration official wondered. "Even if half these things aren't true it's hard to move past it."

It raises questions if Jackson doesn't get confirmed how he'll be able to go back to his previous job as White House doctor.

It also remains unclear why no one from the White House tried to get ahead of the allegations about which they knew, leading some staffers to point the finger at chief of staff John Kelly. The belief among some is that Kelly had reservations about Jackson in the first place and didn't protect him once it became clear his nomination would face hurdles, according one White House official.

Kelly saw all of this happening and relayed the message to Trump on Tuesday that he shouldn't give a full-throttled defense of Jackson given the allegations, a senior White House official. Then, late Tuesday afternoon, Trump and senior staff -- including Kelly -- met with Jackson in the Oval Office and asked him about the allegations. Jackson said the claims were not true and the President gave orders for the White House to go out and fight back forcefully.

Kelly's office did not respond to a CNN request for comment.

One White House aide noted Democratic Sen. Jon Tester's admission that Jackson had received a clean FBI background check "was huge" for the nominee's prospects, which have tanked in the face of withering scrutiny from lawmakers on both sides of the aisle.

Defending the process

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White House sources defended Jackson's formal vetting process .

However, some acknowledged Trump's insistence that Jackson replace Shulkin led to a less thorough vet of Jackson's political suitability for the job -- a potential explanation for the familiar mess the White House found itself in on Tuesday evening.

Additionally, multiple White House sources said senior staff believed Jackson had already been vetted due to his proximity to the President when Trump floated his name internally for the VA position.

The political screening -- which aides described as a process separate from a formal vet -- entailed discussions about Jackson with Trump, among senior aides and with the Presidential Personnel Office, the senior official said. But few of the conversations about Jackson's suitability for the position involved questions about where he stood on various issues related to the VA, the senior official admitted.

"The President had his sights on Dr. Jackson for this position ... and unless somebody was gonna raise a big red flag," the nomination was going to move forward no matter what, the senior official acknowledged.

Sources said Jackson received a formal background check and also made White House staff aware of the baggage in his past before Trump unveiled him as his pick to lead VA.

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A White House aide said Jackson told staff about the likelihood that former employees would raise allegations against him but had explained the context to Trump's team in a way that staffers found satisfactory. The White House aide -- who described Jackson as open and honest during the process -- said the VA secretary nominee rehearsed answers to possible questions about the allegations during a series of practice congressional hearings known as "murder boards," organized by White House staff.

The five practice hearings included questions about the allegations so Jackson could rehearse ways to address them during his upcoming confirmation hearing, the White House aide said. Jackson also relied on the help of two so-called "sherpas," or White House staffers designated to coordinate the paperwork and preparations associated with the confirmation process.

However, the two aides the White House made available to Jackson were relatively low-level within the West Wing hierarchy, raising questions about why a more senior staffer wasn't tasked with overseeing such a high-profile nomination.

A senior White House official said Jackson received a typical vet from the FBI, the Office of Government Ethics and the Presidential Personnel Office, as any appointee would -- even though the nature of Jackson's previous position required him to submit for such vetting periodically. Allegations of drinking on the job and other inappropriate behavior did not show up during the formal vetting process, according to the official.

"The official channels that would render information to us did not flag things here that are going to the Senate," the White House official said.