To some Americans, the biggest news to come out of the presidential debate Tuesday night was not Kamala Harris’s well-received performance. It was Taylor Swift’s endorsement of her minutes after the debate ended.

As celebrity endorsements go, there may not be a bigger one. But could it actually influence the race?

There’s no doubt that Ms. Swift has sway with her legions of fans. There were a total of 405,999 visits to Vote.gov through the link on Ms. Swift’s Instagram story in the 24 hours it was live, far more than the site’s daily average of 30,000 in early September, according to a General Services Administration spokesperson.

But that number alone isn’t particularly useful. It doesn’t tell us whether the visitors were new voters, whether those visits translated into actual registrations or what any new voters will do on Nov. 5.

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The reality is that it’s impossible to know exactly how much of a difference — if any — Ms. Swift will make. But here are some ways to think about the question, after looking at research and speaking with college-age Swift fans in Pennsylvania after the debate.

There are lots of studies on celebrity endorsements, but the evidence that they matter is mixed. And even if they do matter (they are still sought after and welcomed), the actual effects are very hard to measure.

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