By the normal standards of two-party politics, Congress should be on its way to passing some sort of package of gun control laws when it returns from recess after Labor Day. The quick succession of mass shootings this summer in California, Texas, and Ohio has rustled key members into negotiations: Republican and Democratic senators are close to introducing a deal regarding so-called red flag legislation, which would allow authorities to temporarily seize firearms from those who are dangers to themselves or others; on the long-stalled issue of background checks, Republicans and Democrats are working together behind the scenes to try to determine which sort of legislation could win broad support.

This time, even Senate Majority Leader Mitch McConnell has expressed willingness to consider background checks, saying in a Kentucky radio interview last week that background checks will be “front and center” on the agenda when Congress returns, adding that “there is a lot of support for that.” Inaction on gun control was a significant factor costing Republicans suburban House seats in the 2018 midterms, and McConnell is on the defensive protecting his Senate majority in 2020.

But the fate of a deal doesn’t depend on Democratic and Republican lawmakers being able to reach some sort of agreement. “There is a political dynamic here that has to be met,” Democratic Connecticut Sen. Richard Blumenthal told me in an interview. “Republican leadership, directed by the president of the United States, has to support a background-check bill.”

If Donald Trump doesn’t clearly support Congress’ gun control efforts, the negotiations won’t matter. House Democrats, along with eight House Republicans, passed a background-check measure earlier this year, and there could be, as Blumenthal said, “some details that can be tweaked—you know, exceptions for [gun transfers to] family members,” and “maybe some other very narrow” categories that would be exempt from universal background checks. Yet none of those details about exempted categories or other fine print are likely have any effect on the president.

If Donald Trump doesn’t clearly support Congress’ gun control efforts, the negotiations won’t matter.

Instead, gun control rides on the question of how willing the erratic president is to endorse it, and to stick with that endorsement. Trump, in the past couple of weeks has offered hazy, spontaneous support for the idea of expanding background checks. In remarks last week, Trump said that “on background checks, we have tremendous support for really common-sense, sensible, important background checks,” because “we don’t want people who are mentally ill, people who are sick, having guns.”

Trump has, however, toyed with the idea of supporting expanded background checks before. Following the Parkland, Florida, school shooting in early 2018, the president declared that he supported expanded background checks. After a chat with the NRA’s top lobbyist, Chris Cox, though, he pivoted to more industry-approved ideas like arming schoolteachers. Though the NRA has since weakened following power struggles in its leadership—Cox, for example, is gone now—signing a background-check bill would still require Trump to go against his base heading into an election year. And Trump, you may have observed, is not just sensitive to demands from his base. He’s exclusively sensitive to them.

Connecticut Sen. Chris Murphy, one of Congress’ leading gun control advocates, has spoken to the president since the recent shootings, and, Murphy told me, “the president expressed to me that he’s serious about trying to come up with a background-checks proposal that can get Republican and Democratic support.” When I asked him how he gauged the president’s precise level of determination, though, Murphy was at a loss.

“I don’t know,” he said. “I can’t tell. Obviously the president says a lot of things and doesn’t always do the things he says.”

Blumenthal, who is co-authoring the legislation with Republican Sen. Lindsey Graham to “incentivize”—i.e., pay—states to pass “red flag” laws and hopes to see that paired with background-checks legislation, put it another way.

“On what policy would you say there has been an unalterably consistent and unchanging position?” he asked. I offered, “Build the wall!” and he conceded that. In terms of bipartisan legislation, however, when the president has toyed with the idea of compromising—be it on guns, immigration, or infrastructure—his record has always been one of retreating rightward to the base.

“I’ve talked to folks at the White House who say, ‘We’re working on it. We think it’s possible,’ ” Blumenthal said. “But nobody says, ‘We think we have him absolutely committed.’ ”

Mixed signals from the president, or even lukewarm, vague support for expanding background checks, would do as much damage to the Republican whip count in Congress as outright opposition. In order for something to pass, Trump needs to leave no doubt.

“The president,” Murphy said, “has got to endorse a specific measure. It’s not enough for a president to broadly say he’s for meaningful background checks. He ultimately has to put his name on a piece of legislation.”

If Trump starts wavering, Republican senators and members of Congress will pick up the signal, and the best opportunity for passing a background-checks bill in six years will be lost. It wasn’t an auspicious sign, then, that at his New Hampshire rally earlier this week—his first campaign rally since the recent spate of shootings—Trump observed that “there is a mental illness problem that has to be dealt with” but didn’t mention background checks.

“There remains no issue like this in American politics today, where 95 percent of Americans can’t get what they want from Congress,” Murphy told me. “It’s maddening that we’re once again talking about the political difficulty of getting something that is so unbelievably publicly popular done.”