But because of Trump, Republican candidates are talking a lot more about border walls and birthright citizenship than they were earlier in the year. They’re also talking more about China. And this new focus has come at the Iran deal’s expense. Grassroots Republicans still oppose the Iran deal. But they’re not thinking about it nearly as much. Craig Robinson, former head of the Iowa GOP, told me that in the Hawkeye State, Trump has “refocused the summer on what he wanted to talk about.” When I asked Dean Spiliotes, a political scientist at Southern New Hampshire University, about the conversation in his state, he said that, “Among Republicans there’s a lot more energy around immigration and trade and jobs than around Iran.”

I don’t want to overstate the case. The Iran deal would probably be on its way to passage even without Trump. Obama doesn’t need Republican votes. He needs Democratic ones, and most Democrats were not inclined to torpedo the signature foreign-policy initiative of a president of their own party. But to have had any chance, the anti-deal campaign needed to make Iran the dominant issue of the August recess. It needed to replicate the intensity that anti-Obamacare activists generated during the August recess of 2009.

For all the money they’ve spent on ads denouncing the deal, that hasn’t happened. Iran may be issue number one among Republican elites. But it’s not issue number one among the Republican grassroots, let alone Americans as a whole. And Trump is part of the reason why.