WASHINGTON — President Donald Trump is struggling to find a politically tenable perch on gun legislation as he grapples with an issue that has long been a third rail of Republican politics.

Inside the White House, staffers are still trying to determine the specifics of a policy Trump can get behind that would have a shot at passing Congress. White House aides met on Tuesday with staffers for Democratic members of the House Judiciary Committee as well as staffers for Sen. Susan Collins, R-Maine, about straw purchasing legislation, said a senior administration official. (A straw purchase is when someone who can legally buy guns does so for someone who can't.)

Trump also made another call Tuesday to National Rifle Association Chief Executive Wayne LaPierre — one of several conversations the two have had since the Aug. 3 mass shootings in Texas and Ohio.

Now — just a few weeks after coming out publicly for the expansion of background checks — Trump is appearing to waffle on that, with his public comments shifting more toward blaming mental illness rather than easy access to guns for the series of mass shootings that have marked his presidency.

"We have very, very strong background checks right now," Trump told reporters Tuesday. "But we have sort of missing areas, and areas that don't complete the whole circle. And we're looking at different things, and I have to tell you that it is a mental problem, and I've said it 100 times. It's not the gun that pulls the trigger, it's the person that pulls the trigger."

The administration official cautioned that the messaging will most likely continue to be messy and confusing as Trump continues to search for a position to settle on.

Despite Trump's apparent retreat on background checks, White House officials said the president is still pushing for legislation modeled after the failed 2013 bill sponsored by Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa. That measure stops short of a bill passed recently in the House, and doesn’t go as far as universal background checks, something the White House isn’t discussing.

Several factions have emerged in the White House, with different priorities on the issue.

The president’s daughter Ivanka Trump was initially brought in to help reach out to Democrats as well as Republicans on Capitol Hill, and has been seen as a moderating voice. But she is on vacation with her family in Wyoming this week, leaving her out of the daily flow of conversations.

Download the NBC News app for breaking news

Vice President Mike Pence and chief of staff Mick Mulvaney have been focused on changes to the federal death penalty statute that would apply to those who carry out mass shootings.

Trump has emphasized repeatedly that in his view, the problem is mental illness — but it’s still unclear what legislative or executive action could be taken to address that concern.

Trump has to walk a fine line politically — between suburban voters, many of whom want to see action taken on the issue, and his base of Second Amendment advocates — and getting the balance wrong could tip his chances in 2020.

At the moment, the political momentum seems to be toward taking more action rather than less: As many as 89 percent of voters in the most recent NBC/Wall Street Journal poll say they favor congressional expansion of background checks to all firearm sales and transfers — including 75 percent who “strongly” support it.

Trump doesn’t have long to figure out how to thread the legislative and political needle. It is widely expected no significant legislation will be able to be passed in Congress next year when members begin focusing on their re-election prospects. That gives Trump until mid-September, when Congress comes back from recess, until the end of 2019.

For members of Congress, the calculus depends on their district. Democrats run the risk of drawing the ire of gun control groups and facing a primary challenger if they are seen as voting for legislation that doesn’t go far enough.

Meanwhile, Republicans in deep red districts will face their own backlash if they show any signs of supporting moves that might limit gun ownership in some way. It would require Trump to sell any limits on gun ownership to their constituents — something he has yet to show any desire to do.

“Gun control is going to be a tough issue because it pulls in so many other issues that conservatives care about,” Wilson said. “It's not just guns. It's also the Constitution and, in the case of things like 'red flag' laws, the notion of government watch lists.”

After starting off the month advocating for stricter background checks, telling reporters that even Senate Majority Leader Mitch McConnell, R-Ky., was on board, Trump last week didn’t sound like a president trying to sell significant gun control legislation to his base.

Speaking at a campaign rally in New Hampshire on Thursday, the president spoke of areas that might hold solutions to gun violence — but did not mention expanding background checks, instead suggesting red flag laws and a greater focus on mental health. “It is not the gun that pulls the trigger — it is the person holding the gun,” he said. The crowd of supporters erupted in one of its biggest cheers of the night.

Trump himself nodded bluntly to the political reality driving that reaction Tuesday.

"A lot of the people that put me where I am are strong believers in the Second Amendment," he told reporters at the White House. "And I am also."