"This is it. We have got to come together. Because this is the year when we make Barack Obama a one-term president," Bachmann said.
The four-term congresswoman mounted her own bid for the Republican nomination, but ultimately abandoned that effort after she was unable to secure a win in Iowa, her home state.
On the campaign trail, Bachmann was a frequent critic of Romney, chastising the frontrunner on social issues. Romney's refusal to sign a pledge from the Susan B. Anthony List - a prominent conservative anti-abortion group - because frequent fodder for Bachmann on the campaign trail.
But the congresswoman told Huckabee she felt confident that with Romney, evangelicals would have a voice in the White House.
"We will see an open ear from Mitt Romney," Bachmann said. "So far, he has proved to be a person who listens. That's what we need, a nominee who listens."
Bachmann's endorsement could help Romney coalesce some of the Tea Party elements of the party that have so far been reluctant to openly campaign or fundraise on his behalf.
Wednesday night, fellow former rival Rick Perry signaled that he would be endorsing Romney's presidential bid. Newt Gingrich and Rick Santorum have both signaled that they are likely to endorse in the near future, while former contenders like Tim Pawlenty, Herman Cain, and Jon Huntsman already signaled their support for the former Massachusetts governor.