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.