Former U.S. Republican presidential candidate Rick Santorum has formally endorsed his one-time bitter rival Mitt Romney, the party's presumptive nominee.
Santorum's campaign website announced the endorsement early Tuesday morning, nearly a month after Santorum quit his campaign to win the party's nomination for this year's presidential election.
The former Pennsylvanian senator also urged his supporters to back Romney's campaign in an email sent out late Monday night.
"Above all else, we both agree that President Obama must be defeated. The task will not be easy. It will require all hands on deck if our nominee is to be victorious. Gov. Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime," Santorum said.
Santorum said he had felt it "completely impossible" to consider an endorsement until after a meeting to discuss "critical" issues with Romney. He acknowledged the over one hour one-on-one meeting with Romney, who visited him last Friday in Pittsburgh, was "candid" and "productive."
However, Santorum also reminded his supporters of the still-lingering differences between him and the man he once called "the worst Republican in the country."
"The primary campaign certainly made it clear that Gov. Romney and I have some differences. But there are many significant areas in which we agree," Santorum said.
Santorum also signaled he would not leave the campaign trail, looking forward to "working together to defeat President Obama this fall."
Santorum surged as Romney's main rival this year from a relatively low-key figure when he kicked off his campaign. He pulled out of the race in Pennsylvania on April 10, just two weeks before his home state's presidential primary. His departure further cleared the path for front-runner Romney for a seemingly inevitable GOP nomination.