Bachmann says electing Romney will be the country's last chance to keep from "going forward" over a cliff.
The reference is to Democratic President Barack Obama's use of the word "forward" as a campaign slogan.
In a statement announcing her decision, Bachmann says Americans can vote for Obama -- and for more joblessness, higher energy prices, more government controls and failed economic policies. Or, she says, they can vote for Romney and a "new vision of prosperity and liberty."
Bachmann had challenged the former Massachusetts governor for the GOP nomination. She dropped out of the race in January after finishing poorly in Iowa's first-in-the-nation caucuses.