The circle is complete - that is, if you leave out Ron Paul. But fringe candidates don't count so it is significant that Rick Santorum, by endorsing Mitt Romney's candidacy yesterday, became the last of the major candidates who opposed him in the Republican primaries to fall into line.

Politico:

Santorum makes clear that he felt Romney had answered questions about his commitment to principles the former senator cares about at their one-hour meeting in Pittsburgh. And his backing, while expected, comes at a time when Romney is trying to solidify his standing in the party, whose social conservative base remains uncertain about him.

"Above all else, we both agree that President Obama must be defeated," Santorum writes in the email. :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."

Santorum's credentials with social conservatives remain strong, and while Democrats are certain to re-introduce video clips of him talking about Romney, his backing could help the presumptive nominee with the voters he's struggled with most.

In the most recent Battleground Poll, 91% of Republicans support Romney. That's the good news. The bad news is that his support on the social right is tepid, at best. If Santorum can help bring social conservatives into line and go to the polls on election day, he will have earned a huge vote of thanks from Mitt Romney.