The reason Rubio ranks lower in the National Journal rankings is largely because of social issues, on which Rubio scores a 70 and Paul scores a 90. Both rank among the most conservative senators on both fiscal and foreign policy issues.

The two of them are also neck and neck on Heritage Action’s ongoing scorecard, with both of them tied for the second-highest score among current senators (96).

Make no mistake: Rubio, Paul and their advisers are keenly aware of how the other guy is voting on any given issue. Paul’s people think Rubio has been trending more conservative in order to keep pace with Paul, while Rubio’s people emphasize that he has been among the most conservative senators in the chamber over his first two years and remains in good stead with the tea party, which helped him (and Paul) win election in 2010.

Such is the nature of Republican primaries these days. Basically every Republican is afraid of being the next target of the tea party because he or she hasn’t been a down-the-line conservative. And any potential 2016 presidential candidate needs to be careful not to venture into RINO (Republican In Name Only) territory — particularly when the other guy is staying pure.