As Vice President Kamala Harris prepared to visit Michigan on Thursday, the national group that mobilized primary voters to cast “uncommitted” ballots against President Biden over the United States’ support for Israel in the war in Gaza announced that it would not endorse her.

The group, the Uncommitted National Movement, which started in Michigan and has become a national organization, said the decision had come after Ms. Harris failed to respond to its request for her to meet with Palestinian American families who have lost loved ones in Gaza, and for her to discuss the group’s demands for halting arms shipments to Israel. The group said it had asked for a response by Sept. 15.

“Vice President Harris’s unwillingness to shift on unconditional weapons policy or to even make a clear campaign statement in support of upholding existing U.S. and international human rights law has made it impossible for us to endorse her,” the organization said in a statement provided to The New York Times.

Ms. Harris is scheduled to appear at a campaign event on Thursday with Oprah Winfrey in a Detroit suburb, her third trip to Michigan since entering the presidential race. The crucial battleground state has one of the country’s largest populations of Arab American voters, and many of them were among the more than 100,000 people to cast “uncommitted” votes against Mr. Biden in Michigan’s primary election in February. Hundreds of thousands more Americans cast similar protest votes in states across the country.

In its statement, the Uncommitted National Movement urged its members to vote against former President Donald J. Trump, who it said had “bragged about accelerating the genocide against Palestinians and promised to intensify the suppression of pro-Palestinian activism in the U.S.” The group also urged members not to vote for a third-party candidate, saying that could help Mr. Trump win.

The endorsement decision was not a surprise: Tensions between the group and Ms. Harris’s campaign have spilled into public view. Last month, when two Uncommitted leaders briefly spoke with her and asked for a meeting to discuss the group’s demands for an arms embargo, her campaign downplayed the interaction, and her national security adviser made it clear that Ms. Harris did not support an arms embargo.

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