He was #2 in total R primary votes in 2012.
And the #1 guy isn’t running this time.
So I can see his logic, if not his path to winning.
2012 Primaries
Romney 10 million; 45 states by delegates
Santorum 3.9 million; 6 states by delegates
Gingrich 2.7 million; 2 states by delegates
Paul 2 million; 4 states by delegates
The problem is, so many of his votes came after Romney was the winner of the primary. Santorum hung around after others dropped out and got the not-Romney vote in the later primaries after Romney had it won.
About what Huckabee did in 2008 to run up his vote total.
The Santorumn and Huckabee vote totals those years mean very little.