Rick Santorum Endorses Romney, Says Obama Must be Defeated"

Former Pa. Senator sends email to supporters urging them to back Mitt.

The rivalry is over.

Former Pennsylvania Senator Rick Santorum has officially endorsed Republican presidential front-runner Mitt Romney.

In an email sent to his supporters last night, he urged them to back Romney but still reminded them of the clear differences between both he and Romney who he once called the "worst Republican to possibly face Obama."

"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.

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."

Santorum and Romney met late last week and the former senator reportedly landed some of his former staff members jobs on Romney's campaign team. His former campaign manager has signed on with Romney.

[ALSO READ: Santorum Quits Race]

He believes adding staff members to Romney's team will help the former Massachusetts governor focus on issues that Santorum's base is strongly against like Obamacare.

"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 wrote.