Fearmongering on immigration harks back, as I wrote last week, to Trump’s 2016 presidential run, in which immigration fears were central to his approach, beginning with the campaign kickoff, where he said, “When Mexico sends its people, they’re not sending their best … They’re bringing drugs. They’re bringing crime. They’re rapists.”

That was a highly effective strategy for winning over the Republican base, as demonstrated by Trump’s demolition of the rest of the GOP primary field. But it’s not clear that it’s a great strategy for the electorate as a whole. As The Washington Post’s Dave Weigel notes, immigration was a small part of Trump’s closing argument in November 2016, and perhaps not the decisive one.

After all, Trump had beaten the immigration drum throughout the campaign, and by mid-October, he seemed headed for a defeat in the election. His campaign was reeling from the Access Hollywood tape, but he’d also never been leading. Even his top advisers—even Trump himself—expected him to lose. Then came James Comey’s October 28 letter announcing the reopening of the FBI probe into Hillary Clinton’s use of a private email server. As FiveThirtyEight’s Nate Silver and others have concluded, that letter likely lost the election for Clinton. Or, put another way, that letter—and not immigration rhetoric—won the election for Trump.

There’s not a great deal of public polling on Trump and the caravan specifically, but strong majorities consistently disapprove of the president’s handling of immigration, suggesting the issue isn’t a winner for him, even though some of those Americans might feel he’s not going far enough. Meanwhile, polling consistently finds strong support for more dovish immigration policies than Trump’s, including a path to citizenship for recipients of the Deferred Action for Childhood Arrivals program. His hammering on immigration could also inflame Democrats, pushing more of them to the polls.

It seems just as likely that Trump is resorting to immigration at this stage because, even though the economy is strong, his signature policies, like a big tax cut, aren’t popular; because his health-care position is downright unpopular; and because, without Hillary Clinton in the mix, he doesn’t have an effective villain against whom he can campaign.

Just because it’s a desperation play doesn’t mean it won’t work. But if it does, it will do so not by persuading swing voters, but by driving as many Trump-supporting voters to the polls as possible, and neutralizing Democratic advantages on enthusiasm and turnout. The problem is that while Trump’s base remains devoted, it’s also slowly shrinking. Along with others, I have argued for months that Trump’s strategy of only appealing to his hardest-core supporters and not making any effort to expand his coalition is a dangerous strategy, since his base remains a small share of the electorate. The president’s decision to go all in on immigration in the last week of the midterms will provide a crucial test of whether the base is indeed insufficient—or whether he once more knows something the political class doesn’t.