Michigan Democrat Haley Stevens’ shocking last-minute endorsement of Mike Rogers, a Republican Senate candidate, has sparked fierce backlash from members of her party.

Stevens, who narrowly lost in the Democratic primary for Senate to Abdul El-Sayed, announced her support for Rogers after a feud with the El-Sayed campaign over an outside group’s campaign ad that blamed the high cost of living on US support for Israel.

Stevens had said she would appear at a campaign event if he denounced the advertisement, El-Sayed’s campaign disinvited her.

Chuck Schumer, the Senate minority leader, a Democrat from New York, described the move as “a despicable betrayal”.

“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,” Schumer posted on social media.

Nancy Pelosi, a California representative, posted that Stevens had “chosen to dangerously betray Michigan families’ healthcare, freedoms and future with her horrific endorsement”.

Zohran Mamdani, the New York mayor, also said that “working class people’s lives are on the ballot on Tuesday”.

“Haley Stevens just betrayed them by endorsing the very Republican she said she wanted to run against,” Mamdani posted.

Curtis Hertel, the chair the Michigan Democratic party, said in a post that Stevens was not a member of the party “nor will she be in the future”.

A Detroit journalist, Sam Robinson, shared a video in which Hertel was asked why he thought Stevens made the endorsement.

“Obviously there are sour grapes from the election, but there is no reasonable explanation for this. I have known Haley Stevens for a long time, and I know what she has fought for her in her career, and this is the antithesis of all of that,” Hertel said.

Debbie Dingell, a Democratic representative from Michigan, said the state’s residents were focused on the cost of living, healthcare and housing.

“That’s what they’re talking about, not Congresswoman Stevens’ continued efforts to get attention,” she said in a statement.

“In the closing days of this election, we must remember the true choice of this race,” the statement read. “One candidate represents a party that has stoked hatred, gutted anti-discrimination efforts and remained silent in the face of President Trump’s corruption and authoritarianism. The other represents the promise of accountability, affordability, and the fight for a more equitable, just, and democratic future.”

Meanwhile, Trump praised Stevens’ endorsement of Rogers on his platform Truth Social and described her as a “highly respected Democrat”. He continued to support Stevens during a campaign rally Sunday in Dubuque, Iowa, even going so far as to welcome her to join the Republican party.

“I’d love her to join the Republican party, actually, I think that could happen maybe. But she showed great and almost unprecedented loyalty to the great people of Michigan,” Trump said.