Haley Stevens, who lost the Democratic US Senate nomination for Michigan to Abdul El-Sayed, on Sunday broke with her party and endorsed his Republican opponent, Mike Rogers.
Ms Stevens announced her endorsement in a video posted on social media after making a surprise appearance next to Mr Rogers at a church in Detroit.
“As you may have seen, our Democratic nominee for US Senate, Abdul, said that I was no longer welcome in his campaign and that I should stay away,” she said.
“And he's right, and that's why I am endorsing Mike Rogers for the United States Senate today. I don't belong in Abdul's campaign.”
The endorsement comes months after Mr El-Sayed, an Egyptian-American doctor, defeated Ms Stevens in the Democratic primary.
Michigan is set for a critical Senate race on Tuesday. The outcome could help determine which party controls the Senate for the remaining two years of President Donald Trump's term.
Ms Stevens and Mr El-Sayed had initially planned to appear together at a different church in Detroit on Sunday.
But in the lead-up to the event, she demanded that Mr El-Sayed denounce a recent advertisement that blamed rising prices in the US on American military support for Israel.
Jewish-American groups said the advertisement was anti-Semitic and offensive.
Mr El-Sayed said the advertisement was produced by a third party and that his campaign was not involved in its production or content.
In response to Ms Stevens's demand, he accused her of relitigating the primary election and said she was no longer welcome in his campaign.
Mr El-Sayed, a progressive candidate endorsed by Democratic socialist Bernie Sanders, has been an outspoken critic of Israel and has described its war in Gaza as a genocide. He supports ending US military aid to Israel. If he wins, he would be the first Muslim in Congress.
Ms Stevens is an establishment Democrat and a strong supporter of Israel.
US President Donald Trump, who has endorsed Mr Rogers, welcomed Ms Stevens's move.
“Her following is huge, and the level of respect from the people of Michigan, especially our African American and Jewish populations, is virtually unparalleled,” Mr Trump wrote on Truth Social.
“Haley said what few people are willing to say. She showed great and almost unprecedented loyalty to the great people of Michigan with what she did.”
Over the past weeks, Mr El-Sayed has been subject to intense Islamophobic attacks, as well as mockery over his name. Mr Trump and Mr Rogers have accused him of ties to the Muslim Brotherhood and other armed groups in the Middle East, accusations he has repeatedly denied.
In her short video, Ms Stevens said she would remain a “very proud Democrat” but did not believe Mr El-Sayed's campaign represented the values she believed Michigan should stand for.
“Who we are as Michiganders is not hate, and is not what we are getting out from our Democratic nominee right now,” she said.
