Sens. Marco Rubio (R-Fla.) and Rand Paul (R-Ky.) are both strong contenders for the 2016 GOP presidential nomination, and for both of them, there will be a premium on not letting the other guy get to their right.

So far, it’s been a tight battle. According to a trio of new vote ratings released this week, Rubio and Paul are neck and neck when it comes to who has been more conservative. And both, notably, have been among the most conservative senators — much more conservative, as it happens, than another potential 2016 GOP presidential candidate and conservative favorite, Rep. Paul Ryan (R-Wis.).

Below is a chart comparing Rubio, Paul and Ryan in three key 2012 vote ratings released this week by the National Journal, the American Conservative Union and Americans for Prosperity.

The reason Paul ranks lower than Rubio in AFP’s rankings appears to be because he opposed Ryan’s budget when it came up for a vote in the Senate. While Paul joined basically all Democrats and a few moderate Republicans who thought the budget cut too much, he was actually opposing the budget because it didn’t cut enough.

So a case could be made that he was actually more conservative than his AFP rating suggests.

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.

Because of this, it will be very interesting to see on which issues Rubio and Paul align with and depart from each other on over the next two-plus years — starting with immigration reform, on which Rubio has taken a leading role.

How they vote will say a lot about the race that lies ahead.