Rick Santorum has officially endorsed Mitt Romney, and notified his supporters in an email sent Monday night.

In the email, the former presidential candidate cited his Friday meeting with Romney in his hometown of Pittsburgh as a reason he finally got behind the presumptive GOP nominee.

"On Friday, Governor Romney came to Pittsburgh for an over-hour long one-on-one meeting," Santorum said in the email. "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."

The email thanked supporters and was sent in separately to various backers. The letter said the issue of "endorsement did not come up," but the former Pennsylvania senator has heard whether he should weigh a "formal endorsement" or not.

"Thank you for your counsel, it has been most helpful," Santorum wrote in the email. "However, I felt that it was completely impossible for me to even consider an endorsement until after a meeting to discuss issues critical to those of us who often feel our voices are not heard by the establishment: social conservatives, tea party supporters, lower and middle income working families."

Santorum said that in the meeting he "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 said he was "impressed" with Romney's "deep understand" of the presumptive GOP nominee's "deep understanding of this connection and his commitment to economic policies that preserve and strengthen families."

In the lengthy email, Santorum encouraged Romney to "add more conservative leaders as an integral part of his team."

"And 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.

Santorum told his supporters he intends to "keep lines of communication open."

"Above all else, we both agree that President Obama must be defeated," the letter read. "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."

He ended the letter promising his supporter he will have details of his own new venture soon.

Close advisers say it was Santorum's own decision to do it late Monday night. Tuesday, Santorum will appear on "The Tonight Show with Jay Leno," and it was previously thought that Santorum would make his feelings known then.