Romney, a former Massachusetts governor, is all but assured the GOP nomination at this point, with his only remaining rival Ron Paul of Texas, who is stepping down from Congress but is far behind in the delegate count.
In a statement, the Minnesota congresswoman said Romney is "the last chance we have to keep America from going ... over a cliff," Yahoo! News reported. Bachmann was scheduled to make her first appearance with Romney Thursday in Portsmouth, Va.
Bachmann ended her own campaign in January after finishing sixth in Iowa's caucuses but resisted Romney's calls for support, saying she would come around in her own time.
Yahoo! News said Democrats already are using past Bachmann attacks on Romney -- she consistently was Romney's top Republican critic and insisted repeatedly he could not beat President Obama.
Bachmann suggested in a CNN interview last week she was working to bring conservatives into the Romney camp.