Rick Santorum leads the Republican primary field to challenge Senator Bob Casey in 2012, but among better known rivals in the GOP Presidential primary he ranks 5th.

According to numbers from Public Policy Polling, Santorum garners 45% in a hypothetical Senate primary matchup – well ahead of any potential rivals.

Santorum’s performance compared to other GOP presidential contenders is less inspiring.

“Tested against the usual laundry list of potential GOP Presidential candidates he comes in a distant 5th place with 11%. Leading the way is Mike Huckabee at 21%, followed by Sarah Palin at 18%, Newt Gingrich at 16%, and Mitt Romney at 14%. Tim Pawlenty leads the second tier of candidates with 6%, Ron Paul gets 4%, and Mitch Daniels just barely registers at 1%,” writes the pollster.

In the Senate poll, Santoum leads with 45%, Congressman Jim Gerlach rates at 9%, former Governor Mark Schweiker and Congressman Charlie Dent at 8%, Congressman Tim Murphy at 7%, state Senator Jake Corman at 3%, and former Santorum staffer Marc Scaringi and state Senator Kim Ward at 1%.

The Senate poll numbers show what many insiders already know: Bob Casey enjoys relatively stable, net-positive approval in the state, and the eventual Republican nominee is likely to face an uphill fight against Casey’s high name ID.

A previous poll by the same firm showed Casey with a lead of 7-23 points over various prospective challengers.