Congresswoman Haley Stevens, who lost the Michigan Democratic Senate primary to Abdul El-Sayed, endorsed his Republican rival, Mike Rogers, in an extraordinary rebuke of her party’s nominee on the eve of the midterm elections.

Her endorsement came in a video posted to X after she attended an event with Rogers at the predominantly Black Perfecting church in Detroit on Sunday. Stevens’s endorsement was the culmination of an escalating feud between the former primary rivals – primarily over support for Israel – that has roiled one of the most fiercely contested Senate contests of the cycle.

The endorsement was met with immediate pushback from Democrats, who condemned Stevens for her “betrayal”.

Michigan’s Democratic party chair Curtis Hertel said: “Haley Stevens’ decision to campaign with Rogers – a man who has repeatedly sold out Michigan’s working class, wants to pass a national abortion ban, and has pledged to be a loyal vote for Trump’s agenda – shows that she has abandoned everything she claims to care about. We reject the attempts by Stevens and bad-faith actors to divide our party and undermine our momentum, and will not allow these eleventh-hour distractions deter us.”

Some of the people who worked for Stevens’s campaign also reportedly quit following her Rogers endorsement. Her long-time chief of staff, Justin German, and her communications director, Logan Basch, were among those who left, according to CNN correspondent Manu Raju.

German later released a statement on behalf of Stevens’ campaign team endorsing El-Sayed: “As Haley Stevens’ Chief of staff for eight years, I speak on behalf of our team in wholeheartedly supporting Abdul El-Sayed for Senate. This election is too important to sow division or stand on the sidelines. We are shocked and heartbroken by Congresswoman Stevens’ decision to support Mike Rogers.”

Fallout over Stevens’ endorsement of Rogers continued Sunday, with figures from the progressive left stepping up to back El-Sayed.

Bernie Sanders joined the controversy, attacking Stevens for what Democrats view as an extraordinary betrayal after Stevens accepted El-Sayed’s narrow victory over her in Michigan’s Democrat senate primaries.

“Rep. Haley Stevens’ staff is right to resign in disgust,” Sanders posted on X. “They understand that the election in Michigan is far more important than Haley’s fragile ego and that all of us must stand together to elect Abdul, defeat Rogers and end the Republican rubber-stamp control of the Senate.”

Zohran Mamdani, the New York City mayor, said Stevens had “betrayed” working class people “by endorsing the very Republican she said she wanted to run against”.

In his first public comments since Stevens’s endorsement of Rogers, El-Sayed told reporters at a press conference outside of a rally he held on Sunday, the very same rally that Stevens had earlier been disinvited from attending, “At the end of the day, I actually don’t think about Haley Stevens.” He added, “I’m just less interested in the politics of it all than I am in the policy of it all.

“If you are somebody who believes that we should stand up to hate, and you have watched this last campaign, I think we probably would not want to support Mike Rogers and Donald Trump and JD Vance,” he said.

But Stevens’s political pivot earned her praise from Donald Trump, someone whose policies she has an entire page on her website dedicated to denouncing.

Trump, during his rally in Iowa, heaped praise on Stevens for turning against El-Sayed, urging her to join the GOP. “It was an incredible thing,” Trump said. “I’d love her to join the Republican party. I think that could happen.”

Stevens had lost by a close margin in this year’s primary, and for months had refused to openly campaign for El-Sayed, as had his other opponent Mallory McMorrow. But top Democrats had brokered the truce between El-Sayed and Stevens. After the two had sparred viciously in the primary campaign Stevens extended him a tepid endorsement, and this weekend was scheduled to join El-Sayed at a predominantly Black church in Detroit’s North End.

Those plans have quickly unraveled over an issue that divided Stevens and El-Sayed perhaps more than any other: Israel.

Later last week, Stevens, a staunch supporter of Israel, made clear that her appearance alongside El-Sayed was preconditioned on his denouncing of an outside group’s campaign ad that had blamed US support of Israel for the rising cost of living. El-Sayed declined the request, accusing Stevens of needlessly reopening debates from a settled primary.

As the vicious political firestorm has unfolded, Democratic commentators have also pointed out that Stevens’s fiance, Jason Turkish, has been condemning El-Sayed publicly while supporting Rogers. Turkish, who is also vocally pro-Israel, has donated heavily to Democrats in the past, but this year also donated to Republican candidates including Susan Collins.

California governor Gavin Newsom said on the political talkshow 2WAY that Stevens “went another 180 degrees overnight, so something was going on there”.

“She’s been successful at taking our eye off the ball and that’s why I understand party leadership condemning it … This may be curiously the October surprise of sorts,” he added.

Senate majority leader Chuck Schumer also responded negatively to the endorsement. Schumer’s campaign account called it a “despicable betrayal” from Stevens in a statement, but continued to push for unity among Democrats.

“Senate Democrats are united in our support for Abdul El-Sayed, because we know he will fight for the economic interests of working families in Michigan, unlike Mike Rogers, who is a tool of Donald Trump and billionaire donors. We Democrats equally united in our total revulsion at the racist, Islamophobic, and over-the-top false attacks on Abdul by Republicans. These attacks are evidence of the total bankruptcy of the Republican Party today, who resort to ugly and un-American fearmongering because they have nothing to offer on the problems that concern people today.”