WASHINGTON - A leaked document outlining one Trump administration proposal to expand background checks on firearms sales prompted an uproar from the right on Wednesday - underscoring the significant challenges the White House will face on any additional gun restrictions it tries to advance in Congress.

The National Rifle Association, weakened but still influential among conservatives, immediately dismissed the plan drafted by the Justice Department as a non-starter. A White House spokesman denied that the document was a White House product - even though its top legislative official was briefing GOP senators on the plan's details.

Many Republicans who reviewed the specifics of the background checks measure remained lukewarm about it, and a handful of GOP senators who had been directly briefed by Attorney General William Barr on the plan acknowledged that the proposal was incomplete - at best.

"I don't know who leaked it," said Sen. John Cornyn, R-Texas, who discussed the proposal with Barr on Tuesday. "But obviously that wasn't the idea."

For weeks, President Donald Trump has publicly vowed to advance a package of ideas to reduce gun violence after mass shootings in Texas and Ohio galvanized public support for new restrictions and pressure mounted on Capitol Hill to pass substantive legislation addressing the issue.

The administration - led by Barr and Eric Ueland, the White House's director of legislative affairs - began circulating a one-page document on Capitol Hill this week, outlining a new requirement for background checks on all advertised commercial gun sales.

The plan was first reported by The Daily Caller, a conservative news website.

After the plan became public, the White House immediately distanced itself. Trump, who has waffled publicly and in private on various firearms restrictions, has not endorsed any proposal, a White House spokesman said.

"That is not a White House document," said the spokesman, Hogan Gidley, referring to the Justice Department plan. "Any suggestion to [the] contrary is completely false."

Barr, who returned to Capitol Hill on Wednesday to continue briefing senators on the proposal, said in brief remarks to reporters that he was "just kicking around some ideas" in his meetings this week. He declined to get into specifics, saying simply: "There are a number of proposals being considered."

But the plan drew immediate opposition from the NRA, where officials were privately unhappy Wednesday after they reviewed the memo and expressed concern about the White House's handling of the talks, according to one top NRA executive who spoke on condition of anonymity because the official was not authorized to speak publicly.

That concern was quickly relayed to allies on Capitol Hill, according to the executive, who said the outline "reads like a pathway to gun registration and confirms their worst fears."

"This missive is a non-starter with the NRA and our 5 million members because it burdens law-abiding gun owners while ignoring what actually matters: fixing the broken mental health system and the prosecution of violent criminals," said Jason Ouimet, executive director of the NRA's Institute for Legislative Action.

Even before details began to emerge this week, congressional Republicans had been struggling to unite behind a firearms plan, particularly with little direction from the mercurial Trump when it came to what kind of gun measures he would endorse. One senior White House official said public rollout of a guns package was unlikely this week because of dissension within the administration.

Although it was drafted with some White House input, the background check proposal surprised key White House officials including acting chief of staff Mick Mulvaney, who was traveling with the president on the West Coast and not involved in writing the memo, said a senior administration official.

Trump's conservatives allies have told him that he would never get credit for toughening gun laws no matter what he does, and that endorsing additional gun restrictions would only depress enthusiasm among his core base of voters, according to two White House officials.

Meanwhile, Ivanka Trump, the president's daughter and White House senior adviser, has been the biggest force within the administration lobbying for a plan that would expand background checks, according to a White House official familiar with the internal dynamics - prompting fears from elsewhere in the White House that the president himself would go for it because she is pushing it.

The White House officials spoke on the condition of anonymity to freely discuss private deliberations.

A broad swath of Republican senators also hails from rural states, where the culture of gun ownership runs deep and where voters are averse to any measure that would be seen as infringing on Second Amendment rights.

Sen. Kevin Cramer, R-N.D., said the proposal that was being pitched by the administration appeared similar to legislation drafted by Sens. Patrick Toomey, R-Pa., and Joe Manchin, D-W.Va., in 2013 that failed to advance in a Senate controlled by Democrats.

"Expanding background checks has always been viewed by a lot of our constituents, my constituents, as a slippery slope and probably something that's not all that helpful," Cramer said. "I think most people I know, in North Dakota anyway, prefer dealing with existing law and enforcing it better."

Sen. Josh Hawley, R-Mo., a favorite of the right who also met with Barr earlier this week, declined to take a position on the administration's proposal and also cautioned that the plan was premature.

"My question to the attorney general [on Tuesday] was, 'What is the president going to support? What is the president going to put forward?' " Hawley said. "And I don't know the answer to that yet."

The lack of enthusiasm on the right didn't mean Democrats were on board, either.

"It is strange to be shopping [a proposal] that doesn't have the president's imprimatur," said Sen. Chris Murphy, D-Conn., one of the most prominent advocates of gun control in Congress. He said he had yet to endorse or oppose the plan drafted by Barr.

The two senators who appeared even remotely warm to the administration's offer were Manchin and Toomey - who, along with Murphy, met privately with Barr in Toomey's office on Capitol Hill later Wednesday.

Still, the three men - each of whom has spoken with the president repeatedly in recent weeks - cautioned that nothing had been finalized.

"I think he has advanced some ideas that are very constructive, very thoughtful and could go a long way toward expanding background checks in a way that poses absolutely minimal inconvenience on law-abiding citizens and increases the chances that we would keep guns out of the hands of dangerous people who shouldn't have them," Toomey said. "I think there's some details that still need to be worked out."

Yet whatever is finalized among senators may fall short in the Democratic-led House, where the leadership had demanded nothing short of a universal background checks bill that passed earlier this year but faces a veto threat from the White House.

Rep. Mike Thompson, D-Calif., chairman of House Gun Violence Prevention Task Force, said he would review the administration's plan but noted that Trump has flirted with embracing expanded background checks only to back away.

Ueland, speaking to reporters on Capitol Hill, dismissed such criticism.

"I don't put any credence [in] that at all," he said. "In fact, it's important for people to realize that this president is open to and seeks input across a wide array of perspectives as he comes to conclusions about policy decisions, which you all witness, cover, talk about on television, put in print and tweet about nonstop."

When asked who drafted the memorandum, Ueland just laughed.

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The Washington Post’s Robert Costa, Erica Werner, Mike DeBonis and Karoun Demirjian contributed to this report.