Haley Stevens throws her support behind Republican Senate candidate Mike Rogers in Michigan. ( Reuters: Rebecca Cook )
A Democratic congresswoman has announced she would support the Republican candidate in a key senate race.
The contest in Michigan is expected to be among the closest in the midterm elections.
Haley Stevens's decision has been described as an "extraordinary betrayal".
The Democrats' messy Michigan Senate campaign has again been marred by friendly fire, with defeated primaries candidate Haley Stevens breaking ranks and endorsing the Republican nominee in Tuesday's midterm elections.
Analysts are expecting the Democrats to perform well against Donald Trump's Republican Party in many parts of the United States during this week's votes, which could reshape the Congress.
But the Democrats' efforts to retain their Senate seat in Michigan have been disrupted by an internal feud between the party's candidate Abdul El-Sayed and Ms Stevens, his rival in the primaries, which has included allegations of antisemitism, racism and xenophobia.
The pair's relationship plumbed new depths at the weekend, when Mr El-Sayed announced Ms Stevens was no longer invited to join him at a church service in Detroit on Sunday, local time.
The campaign event was supposed to be a show of party unity after several months of jibes, but it ended up exposing how deep their rift had become.
Mr El-Sayed justified the decision by saying he did not want any "distractions", although any hope of that crumbled when Ms Stevens turned up at a different church with the Republican candidate Mike Rogers on Sunday morning.
Later that day, she went a step further and held a joint press conference with Mr Rogers in which she said she was a "proud Democrat".
"This is a very close election. We all know that," she said.
"But Abdul made very clear that I wasn't welcome in his campaign, that he didn't need me there that he didn't want me and at some point you gotta look at who’s standing for country."
The Democrats need to retain all of their current Senate seats, and flip four more, in order to wrest control of the chamber.
Chuck Schumer, the Senate minority leader and one of the most senior Democrats in the country, described Ms Stevens's decision as a "despicable betrayal".
In a social media post, Mr Trump praised Ms Stevens: "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."
Despite president's plummeting approval ratings around the country, the Republicans are hopeful of a strong showing in Michigan.
Mr Rogers is considered a moderate candidate, and the party has attempted to style Mr El-Sayed, who is attempting to become the first Muslim elected to the US Senate, as a radical socialist (a label he denies).
Mr El-Sayed's combative style has earnt him plenty of admiration, particularly by left-wingers who believe the Democrats should do more to stand up to Mr Trump, although it has also sparked criticism from opponents and from some within his own party.
Mr El-Sayed is an outspoken critic of Israel and the US military and financial support for the country.
Abdul El-Sayed's supporters believe Democrats should do more to stand up to Donald Trump. ( Reuters: Rebecca Cook )
Ms Stevens, a four-term congresswoman who gave up her seat to run for the Senate, is widely described as being among the most pro-Israel Democrats on Capitol Hill, and has accused Mr El-Sayed of not denouncing Islamic extremist groups strongly enough.
She has accused Mr El-Sayed of being antisemitic and spreading "hate", which he denies. Mr El-Sayed has argued he has faced racist and Islamophobic attacks from Republicans over the course of the campaign.
Several of her staff reportedly resigned after her decision to back Mr Rogers.
Speaking on Sunday, Mr El-Sayed batted away questions about Ms Stevens's decision.
"I’m not in the business of driving clicks for you. I’m in the business of trying to solve problems for the people of my state," he told reporters.
"None of this matters. The rest of this is just a distraction."
Mr Rogers said he was "honoured" to have Ms Stevens support.
"This is courage in politics in America today," he said.
