Before Donald J. Trump traveled to Philadelphia for this week’s debate, he invited one of the internet’s most polarizing figures along for the ride.

Laura Loomer was backstage with the Trump entourage while Mr. Trump squared off against Vice President Kamala Harris. She was in the spin room with the former president immediately afterward. And the next day, she flew with him to New York City and Shanksville, Pa., to commemorate the anniversary of Sept. 11.

A far-right activist known for her endless stream of sexist, homophobic, transphobic, anti-Muslim and occasionally antisemitic social media posts and public stunts, Ms. Loomer has made a name for herself over the past decade by unabashedly claiming 9/11 was “an inside job,” calling Islam “a cancer,” accusing Ron DeSantis’s wife of exaggerating breast cancer and claiming that President Biden was behind the attempt to assassinate Mr. Trump in July.

Just two days before the debate, Ms. Loomer, 31, posted a racist joke about the vice president, whose mother was Indian American. Ms. Loomer wrote on X that if Ms. Harris won the election, the White House would “smell like curry.”

For many observers, including some of Mr. Trump’s most important allies, the Republican presidential nominee’s choice at a critical moment of the campaign to platform a social-media instigator, albeit one with nearly 1.3 million followers on X, was stunning.

“The history of this person is just really toxic,” Senator Lindsey Graham of South Carolina, a Trump ally, told a reporter for HuffPost on Thursday. “I don’t think it’s helpful at all.”