Look what she had to do.

Taylor Swift, one of the most famous icons of American pop culture with a huge fan base around the world, endorsed Vice President Kamala Harris late Tuesday after she participated in the debate against former President Donald J. Trump.

Ms Swift’s endorsement, delivered minutes after Ms Harris and Mr Trump left the debate stage in Philadelphia, offers Ms Harris an unprecedented celebrity backer and a huge adrenaline boost for her campaign, particularly among the younger voters she is hoping to appeal to.

“Like many of you, I watched the debate tonight,” Ms Swift wrote on Instagram to her 283 million followers. “I will be voting for Kamala Harris and Tim Walz in the 2024 presidential election. I’m voting for @kamalaharris because she fights for the rights and causes that I believe need a champion to represent her.”

She captioned her post, “Childless cat lady,” a reference to comments made by Trump’s running mate, Senator JD Vance of Ohio, about childless women. The photo accompanying her post showed her holding a furry cat named Benjamin Button, her pet ragdoll.

Ms Swift’s endorsement was highly anticipated by Democrats. The singer has expressed regret for not being more outspoken about her opposition when Trump first ran for office in 2016. Since then, she has taken a more political stance, speaking out on issues such as abortion access. But the exact timing of Tuesday’s endorsement was a surprise: Ms Swift endorsed Joe Biden on October 7, 2020, shortly before the election.

The impact of Swift’s support may be difficult to quantify, but her ability to get her followers to register to vote became clear just last year. In a short post on her Instagram account in 2023, Swift encouraged her then-272 million supporters to cast their ballots, including a link to the Vote.org website.

The website later reported 35,252 new registrations that day, a significant increase from the previous year and a particularly significant increase in a year without elections.

On Tuesday, Ms. Swift added a similar link to Vote.gov to her Instagram story.

In her post supporting Harris, Swift also mentioned her “fears” about artificial intelligence, pointing to content generated by the technology that falsely suggested she supported Trump, which the former president promoted on social media, and underscoring fears that Americans wouldn’t know where she really stood if she didn’t speak out.

“It really revived my fears about AI and the dangers of spreading misinformation,” Ms Swift wrote. “It made me realize that as a voter, I need to be very transparent about my actual plans for this election. The easiest way to combat misinformation is with the truth.”

Mr Trump called Ms Swift “liberal” but “beautiful” and also praised a friend of hers, Brittany Mahomes, the wife of Kansas City Chiefs quarterback Patrick Mahomes. Some of Ms Swift’s fans had encouraged her to make her support for Ms Harris public to provide a contrast after it appeared Ms Mahomes had liked one of Mr Trump’s social media posts.

A star musician in country and pop music for nearly two decades, Ms. Swift is one of the few celebrities with broad appeal and the ability to cut through a crowded media environment. Her romance with Chiefs star tight end Travis Kelce has captivated the world of football and culture, and she is in the final stretch of a stunning international tour that has sold out stadiums around the world.

Karoline Leavitt, a spokeswoman for the Trump campaign, dismissed the support as “further proof that the Democratic Party has become the party of the wealthy elite.”

In 2020, Swift’s support for Biden and Harris sparked significant backlash from conservatives who urged her to keep her music career apolitical.

Four years later, her growing political involvement led to feverish speculation among Democrats about whether and when she would endorse Ms Harris. Last month, some fans were convinced she was signaling her support by posting a photo of one of her backup dancers in silhouette, bearing a resemblance to the vice president.

Mrs Harris, for her part, has incorporated pop music into her election campaign.

Her rallies were as much like concerts as they were political events: hip-hop stars like Megan Thee Stallion performed and DJs pumped up crowds of thousands of dancing people before the vice president took the stage to Beyoncé’s song “Freedom.” (Many Democrats had hoped Beyoncé would perform at the convention in Chicago, but rumors of her attendance proved false.)

By comparison, Mr Biden’s rallies were small and low-energy, often climaxing with the sound of high school drum music.

Polls show Harris doing much better than Biden among younger voters, a key contributor to her resurgence in the polls that has allowed her to catch up with Trump. Swift’s support for her campaign is a reflection of that popularity.

In her endorsement, Ms Swift added that she was “encouraged and impressed” by Ms Harris’ choice of Governor Tim Walz as her running mate, saying that Mr Walz had “advocated for LGBTQ+ rights, IVF and a woman’s right to her own body for decades.”

Shortly thereafter, Mr. Walz appeared on MSNBC, where anchor Rachel Maddow read Ms. Swift’s entire endorsement statement to him.

“I am especially incredibly grateful to Taylor Swift,” Walz said, placing his hand on his heart. “I also say this as a cat owner – a cat owner like me.”

Ms. Swift has long urged her followers to fulfill their civic duty. On Election Day 2016, for example, she posted a picture of herself standing in a long line. Her fans mistook the photo for a cryptic endorsement of Hillary Clinton.

It wasn’t until 2018 that Ms. Swift made her first official foray into politics, endorsing Democrat Phil Bredesen over Republican Marsha Blackburn in a Senate race in Tennessee. (Nashville is the country music capital of the country, and Ms. Swift maintains a residency there.)

In this deep red state, Ms. Blackburn won the race by more than 10 percentage points.

In an interview with Vogue in 2019, Ms. Swift indicated that she wanted to be more vocal in her support of Mrs. Clinton, but feared her support could backfire. She said she feared that Mr. Trump might try to “weaponize the idea of ​​celebrity endorsement against her and Mrs. Clinton.” Ms. Swift also expressed concerns that public criticism of her at the time could be unfairly applied to Mrs. Clinton as well.

Ms. Swift continued: “In the summer before this election, all people were saying was, ‘She’s calculating. She’s manipulative. She’s not what she seems. She’s a snake. She’s a liar.’ These are the exact same insults people were throwing at Hillary. Would I be a support or a liability?”

Jess Bidgood, Michael M. Grynbaum, Julia Jacobs, Jazmine Ulloa And Michael Gold contributed to the reporting.