Will Taylor Swift’s recent endorsement of Kamala Harris affect the US presidential election? It certainly can. I’ve been a researcher of celebrity involvement in American politics for several years. The research shows that for most American voters a celebrity endorsement won’t make a difference. Voters have strong partisan allegiances and very few Americans are in the undecided category. Recent polling from YouGov suggest that about 5% of Americans are undecided between Trump and Harris. However, these 5% can be the difference between winning and losing. The same YouGov poll shows Trump and Harris tied at 45% each. The research I have seen suggests that about 11% of US adults can be persuaded by a celebrity endorsement and about 19% of young adults say the same. The impact of a celebrity endorsement is strongest among irregular or new voters. People that are newly interested in political issues or those who are typically preoccupied by other concerns, such as celebrity gossip, are the people for whom Swift’s influence could matter. At this point both Trump and Harris are trying to attract the marginal undecided voters, while simultaneously energising their core supporters. Both the Trump and Harris campaigns have received many celebrity endorsements over the past several months. Kamala Harris has been endorsed by Jeff Bridges, Cher, Jamie Lee Curtis, Viola Davis, Mark Hamill, Spike Lee, John Legend, Demi Lovato, Megan Thee Stallion, Amy Schumer, Barbra Streisand, Kerry Washington and Bradley Whitford. Donald Trump has been endorsed by Jason Aldean, Roseanne Barr, Hulk Hogan, Kid Rock, Amber Rose and Jon Voight. If Swift were to make an appearance at a Harris rally, there is no doubt she would electrify the crowd, and it would become a major story. By bringing attention to candidates and issues, celebrities can keep the politician in the news cycle. They can also make political activism the hip thing to do. Politicians are generally disliked, so if they can manage to be “cool” or “based” or “brat” in the eyes of a popular celebrity they can distance themselves from the stigma associated with “establishment” politicians. Celebrities do need to be careful about expressing their political views. Most Americans say they would be less likely to be a fan of a celebrity who supports a politician they don’t like. Basketball star Michael Jordan famously said “Republicans buy shoes too” after refusing to endorse a black Democratic candidate in North Carolina. Staying neutral makes economic sense for entertainers. But some things are more important than money. Celebrities are people too. They have opinions, they want to make a difference and, unlike the rest of us, they have a platform that allows them to reach millions of people who are interested in what they have to say. Some research has suggested that Barack Obama gained an additional 1 million votes in 2008 because of Oprah Winfrey’s endorsement of him. Both Winfrey and Swift have an enormous following, and if even a small percentage of them took their political cues from the entertainers that could be just enough to swing the election from one candidate to the other. Because of the peculiar US system known as the electoral college, where one wins the presidency not because one is more popular but because one won the correct combination of differently weighted states, a few thousand votes in a handful of key swing states can make all the difference. No doubt several political scientists in America are thinking about clever ways to measure the “Swift effect” in this year’s election. Richard T Longoria is associate professor of political science at the University of Texas Rio Grande Valley and the author of Celebrities in American Elections: Case Studies in Celebrity Politics