WASHINGTON (AP) — Democrats in Congress are trying to pass the first major gun control legislation in more than two decades with the support of President Joe Biden, who said Thursday that it is “long past time” to do so. But they are confronting a potentially insurmountable question over what rules should govern private sales and transfers, including those between friends and extended family, as they seek Republican votes.

A bipartisan Senate compromise that was narrowly defeated eight years ago was focused on expanding checks to sales at gun shows and on the internet. But many Democrats and gun control advocates now want almost all sales and transfers to face a mandatory review, alienating Republicans who say extending the requirements would trample Second Amendment rights.

The dispute has been one of several hurdles in the renewed push for gun-control legislation, despite wide support for extending the checks. A small group of senators have engaged in tentative talks in the wake of recent mass shootings in Atlanta and Colorado, hoping to build on bipartisan proposals from the past. But support from at least 10 Republicans will be needed to get a bill through the Senate, and most are intractably opposed.

Connecticut Sen. Chris Murphy, the lead Democratic negotiator on guns, said he’s been on the phone with Republican colleagues every day “making the case, cajoling, asking my friends to keep an open mind.” In an interview with The Associated Press, he said he’d discussed the negotiations personally with Biden on Thursday and that “he’s ready and willing to get more involved" in the talks.

“I think it’s important to keep the pressure on Congress,” Murphy said.

While pushing lawmakers to do more, Biden announced several executive actions to address gun violence, including new regulations for buyers of “ghost guns” — homemade firearms that usually are assembled from parts and often lack traceable serial numbers. Biden said Congress should act further to expand background checks because “the vast majority of the American people, including gun owners, believe there should be background checks before you purchase a gun.”

Still, the gulf between the two parties on private gun transactions, and a host of other related issues, has only grown since 2013, when Senate Democrats fell five votes short of passing legislation to expand background checks after a gunman killed 20 students and six educators at Sandy Hook Elementary in Connecticut. That defeat was a crushing blow to advocates who had hoped for some change, however modest, after the horrific attack.

The compromise legislation, written by Democratic Sen. Joe Manchin of West Virginia and Republican Sen. Pat Toomey of Pennsylvania, flamed out again in 2016, after a mass shooting in Orlando.

Starting anew with Biden in the White House, Democrats are focused on legislation passed by the House that would expand background checks to most sales and increase the number of days a buyer has to wait if a background check is not finished. Murphy said there may not be an appetite to pass those House bills without changes, but after talking to most Republicans over the last several weeks he says he has “reason to believe there is a path forward.”

Under current law, background checks are required only when guns are purchased from federally licensed dealers. While there is agreement among some Republican lawmakers, and certainly among many GOP voters, for expanding the background checks, the issue becomes murkier when the sales are informal. Examples include if a hunter wants to sell one of his guns to a friend, for example, or to his neighbor or cousin — or if a criminal wants to sell a gun to another criminal.

Democrats say private sales can lead to gun trafficking.

“What a lot of people don’t know is that people engage in private sales but they do it constantly,” said California Rep. Mike Thompson, the lead sponsor of the House bill. “They could sell hundreds of guns a year, quote-unquote, privately.”

Republicans say that requiring a background check for a sale or transfer between people who know each other would be a bridge too far. Toomey says Democrats won’t get 60 votes if they insist upon it.

“Between the sales that already occur at licensed firearms dealers, all of which require a background check, and what we consider commercial sales — advertised sales, gun shows and on the internet — that covers a vast, vast majority of all transactions,” Toomey said. “And it would be progress if we have background checks for those categories.”

Manchin also opposes the House bill requiring the universal background checks. “I come from a gun culture,” Manchin said in March. “And a law-abiding gun owner would do the right thing, you have to assume they will do the right thing.”

Murphy hinted that Democrats might be willing to compromise somewhat on the scope, saying he is committed to universal background checks, but he won't “let the perfect be the enemy of the good.”

The House bill would apply background checks to almost all sales, with certain exceptions — including an inheritance or a “loan or bona fide gift” between close family members. Other exemptions include temporary transfers to people who need a firearm to prevent “imminent death” or are hunting.

The Manchin-Toomey compromise in 2013 included additional measures to lure support from Republicans and the National Rifle Association, which eventually opposed the bill. Those included an expansion of some interstate gun sales and a shorter period for background checks that weren't completed — a deal-breaker for Democrats and gun control groups today.

Christian Heyne, vice president of policy at Brady Campaign, said the advocacy groups “will not allow allow for gun industry carveouts to be part of the next piece of legislation that the Senate votes on.” The bill should be “fundamentally different” than eight years ago, he said, since their movement has “only grown in momentum and strength.”

Senate Majority Leader Chuck Schumer has said he will bring gun legislation to the floor with or without 60 votes, but he has tasked Murphy with trying to reach a deal first. Murphy says that if they could win enough votes on the background checks bill, it could pave the way for even tougher measures like the assault weapons ban Biden has backed.

But most Republicans are unlikely to budge. And the NRA, while weakened by some infighting and financial disputes, is still a powerful force in GOP campaigns.

In a statement, the NRA said the House bills would restrict gun owners’ rights and “our membership has already sent hundreds of thousands of messages to their senators urging them to vote against these bills.”