The pop superstar Beyoncé will appear on Friday with Vice President Kamala Harris at a rally in Houston about abortion rights, according to two people briefed on the plans, adding another gold-plated name to the list of celebrities who have thrown their support behind Ms. Harris, including Taylor Swift, Oprah Winfrey and Robert De Niro.

Beyoncé, a Houston native, is one of the most commercially successful musicians of the 21st century and has hundreds of millions of followers on social media, including many Black voters and younger Americans. Her music has already become familiar at Ms. Harris’s large campaign rallies: The vice president enters and exits the stage to “Freedom,” a song from Beyoncé’s 2016 album, “Lemonade.”

It was not immediately clear if Beyoncé planned to endorse Ms. Harris on Friday, something Democrats had long hoped for. But her appearance, which was earlier reported by The Washington Post, will be seen as a clear show of support. The country music legend Willie Nelson will also attend the rally.

Ms. Harris is speaking in Houston to highlight Texas’ almost total abortion ban — and to castigate former President Donald J. Trump, who appointed three of the Supreme Court justices who helped overturn Roe v. Wade. Mr. Trump is almost certain to win Texas, but Ms. Harris will also have the chance to elevate the long-shot bid of Representative Colin Allred, a Democrat challenging Senator Ted Cruz, Republican of Texas.

She has made the concept of freedom a major theme of her run for the White House, arguing that Democrats embody that ideal far more than do Republicans, who have passed a series of abortion bans in conservative states. This summer, Beyoncé granted permission to the Harris campaign to use “Freedom,” a hard-charging anthem of empowerment, according to CNN.

Between Beyoncé and Ms. Swift, who endorsed Ms. Harris minutes after her debate against Mr. Trump in September, the vice president now has the backing of two of the world’s most popular musicians, who carry particular influence with younger people.

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