Last night, the GOP took another step towards unifying behind its presumptive nominee for President, Mitt Romney.  Rick Santorum, who emerged late in the cycle to become Romney’s toughest competitor, formally endorsed him as the nominee in an e-mail sent out to supporters:

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.” …

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 the presumptive GOP nominee’s “deep understanding of this connection and his commitment to economic policies that preserve and strengthen families.”

Santorum is scheduled to appear on The Tonight Show with Jay Leno this evening, and ABC’s Shushannah Walshe says that Santorum was expected to make a commitment at that time.  Santorum apparently decided to announce the endorsement as soon as he made the decision.  That may take a little of the drama out of tonight’s appearance, but it also may draw more interest as people tune in to hear Santorum’s explanation of the endorsement.  Leno will no doubt tease Santorum about his campaign attacks on Romney as the worst Republican to put up against Obama in the fall, and Santorum’s responses will make some news, undoubtedly.

What does this mean for the race?  First, the ad cutters at Team Obama will be busy this week with a new project, but that was going to happen anyway.  Romney should solidify his support among the social-conservative base of the party, and perhaps even get a little more enthusiasm from it.  It sounds as if Santorum could become a surrogate for Romney among the constituencies where Santorum’s message resonated, which would help Romney most in the Rust Belt states where the presidency could be won or lost.  The relative rapidity of the endorsement — last Friday was only their first meeting since Santorum’s withdrawal — puts an end to stories of Republican disunity far ahead of the convention.

It’s hardly a game changer, but it resolves a few open issues for Team Romney well ahead of schedule, and that is important for both fundraising and focus.