Rep. Michele BachmannMichele Marie BachmannBoehner says he voted for Trump, didn't push back on election claims because he's retired Boehner: Trump 'stepped all over their loyalty' by lying to followers Boehner finally calls it as he sees it MORE (R-Minn.) will endorse Mitt Romney's presidential bid on Thursday, a campaign source told The Hill.

A one-time front-runner in the GOP presidential race, Bachmann didn't endorse any candidate after dropping out of the race in January following a 6th-place finish in Iowa's caucuses. Most expected if she did endorse while the primary was still in full swing, it would be for Rick Santorum, who snagged her spokeswoman and many of her supporters after she bowed out.

Bachmann was a harsh critic of Romney during the GOP debates, persistently questioning his conservatism — especially on social issues such as abortion. She repeatedly urged conservatives not to settle for a moderate candidate, an unambiguous reference to Romney.

But while Bachmann has remained coy about her endorsement plans since leaving the race, she has also softened her rhetoric on Romney, and insisted that she would be a force for GOP unity behind the party's eventual nominee.

Echoing similar sentiments by Santorum and other former candidates, Bachmann has emphasized that any Republican would be better than President Obama, allowing her to avoid contradicting herself with her endorsement for Romney, which was first reported by the National Review.

"We will see an open ear from Mitt Romney," Bachmann told CNN in late April. "So far, he has proved to be a person who listens. That's what we need — a nominee who listens."

One of Bachmann's other former opponents in the GOP primary — Newt Gingrich — was similarly noncommittal Wednesday as he announced he was suspending his presidential campaign. Gingrich said he preferred Romney to Obama, but stopped short of offering the presumptive GOP nominee his full endorsement.

Bachmann is up for reelection in the House in the fall, and faces multiple Democratic opponents in the redrawn Minnesota district where she is running. She had more than $1 million in campaign debt stemming from her presidential bid as of the end of March.

- This story was posted at 6:13 p.m. and has been updated.