Rick Santorum endorsed presumptive GOP nominee Mitt Romney in an email to supporters sent late Monday night.

The one-time Republican presidential hopeful and former Pennsylvania senator said “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.” 

{mosads}”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. Governor Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime,” said Santorum.

While his backing could shore up Romney’s position with the GOP’s conservative base, some could view it as a lukewarm endorsement of the party’s eventual nominee.

Santorum’s email was sent at 11 p.m. ET and the campaign made no mention of a public appearance with Romney. His endorsement did not appear until the thirteenth paragraph of the sixteen-paragraph email.

Santorum waged a tough campaign against Romney for the party’s nomination, working to rally support from the party’s right by presenting himself as the conservative alternative to Romney. 

His campaign topped expectations, as he won 11 states, but he was unable to overcome Romney’s edge in fundraising, organization and eventually delegates.

Santorum said his decision to endorse Romney came after a meeting in Pennsylvania.

“On Friday, Governor Romney came to Pittsburgh for an over-hour long one-on-one meeting. The conversation was candid, collegial and focused on the issues that you helped me give voice to during our campaign; because I believe they are essential ingredients to not only winning this fall, but turning our country around,” Santorum said.

“During our meeting I felt a deep responsibility to assess Governor Romney’s commitment to addressing the issues most important to conservatives, as well his commitment to ensuring our appropriate representation in a Romney administration,” he added.

Santorum said he encouraged Romney to strengthen his links with the party’s conservatives, who were slow to rally behind his candidacy.