PHILIP ELLIOTT, Associated Press

WASHINGTON - Former presidential hopeful Rick Santorum urged his supporters to back fellow Republican Mitt Romney's campaign Monday in a late-night email that ignored that Santorum once calling Romney the "worst Republican in the country" during their bitter contest.

Santorum, a former senator from Pennsylvania and a newly minted celebrity among conservatives, said that the one-time rivals would unite to deny President Barack Obama's re-election. But in a nod that the wounds had not yet healed, he reminded his supporters of the deep differences between the two and that misgivings had not yet abated.

"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 wrote, citing common ground in economic, social and foreign policy.

He added: "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."

It was a sharp turnabout from what he had to say about Romney in Wisconsin: "Pick any other Republican in the country. He is the worst Republican in the country to put up against Barack Obama."

Romney visited Santorum for more than an hour on Friday at the Pittsburgh office of Santorum's longtime strategist. The session covered many of Santorum's concerns about Romney's campaign, especially the sincerity of his vow to repeal Democrats' national health care law that was modeled on one Romney signed into law as Massachusetts governor. Those worries, it seems, were assuaged during their private session that ended without Santorum's public backing.

"And while I had concerns about Gov. Romney making a case as a candidate about fighting against Obamacare, I have no doubt if elected he will work with a Republican Congress to repeal it and replace it with a bottom up, patient, not government, driven system," Santorum said in an email that allowed him to bypass a public event with the two standing arm in arm.

Santorum also urged Romney's campaign to incorporate some of Santorum's former aides and advisers to ensure conservatives are represented. That too seems to have begun. Santorum's former campaign manager has signed on with Romney to help his outreach to conservatives, a voting bloc that was skeptical of Romney's changed positions on bedrock issues such as abortion rights.

"You can be sure that I will work with the governor to help him in this task to ensure he has a strong team that will support him in his conservative policy initiatives," Santorum said.