Michigan Democrat Haley Stevens’ shocking last-minute endorsement of Mike Rogers, a Republican Senate candidate, in the run-up to the elections 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.

Following the extraordinary public rupture on Sunday, El-Sayed’s campaign said it raised more than $2.4m, a fundraising record in the final days of a race that could determine control of the US senate. The Democrat’s campaign said more than 26,000 people contributed for the first time on Sunday.

Chuck Schumer, the Senate minority leader, a Democrat from New York, described Stevens’ endorsement of Rogers 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.

Stevens refused to clearly explain what led to her endorsement of Rogers in an interview with CNN on Sunday evening. “I’m just casting my one vote this way,” Stevens said, while insisting she still wanted Democrats to win the majority on election day.

“I am not trying to upset anyone,” she added. “I’m just being honest with my constituents and what I’m hearing from people, and in fact, I’ve been hearing from a ton of Dems, Anderson, who said: ‘Thank you, this is how we felt; we haven’t liked this campaign; it’s painful.’”

Cooper pressed Stevens on her decision, questioning whether being disinvited from El-Sayed’s campaign event triggered her endorsement of Rogers. At one point, Stevens refused to answer the question, allowing several seconds of silence to air.

Stevens has continued to defend her decision, as she faces accusations of betrayal from Democrats, including calls for her expulsion from the House caucus. Several members of her congressional staff resigned in protest. On Monday, she drew a parallel with Pennsylvania Senator John Fetterman, a one-time progressive who now frequently breaks with his party, especially over Israel.

“You see a Democrat from Pennsylvania, you know, his name’s a lightning rod in the party. His name’s Fetterman,” she told conservative commentator Scott Jennings. “They say the same thing about him.”

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.

On MS Now on Sunday, El-Sayed encouraged undecided Michigan voters to stand with him to protect democracy in the United States. “The divide in our politics isn’t left versus right; the divide in our politics is the people who are locked out versus the people who are locking them out,” El-Sayed said.

Speaking to Fox News on Monday, Rogers praised Stevens for her actions. “I know Democrats are thumping their chest and saying she has done something other than courageous,” he said. “It was nothing but courageous.”

While some Democrats and Jewish leaders also condemned the advertisement for El-Sayed, congressional nominees Brad Lander and Daniel Biss, both of whom are Jewish and Democrats, issued a statement on Sunday encouraging voters to support El-Sayed. They did not mention Stevens.

“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 on 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.

When asked about Trump praising her, Stevens told CNN: “Well, I impeached him twice,” before criticizing Republicans for being “really focused on just letting Trump be the dictator”.