Bob Casey and Dave McCormick are running for the U.S. Senate seat in Pennsylvania alongside three third-party candidates.

The neck-and-neck contest between Sen. Bob Casey (D-PA) and Republican challenger Dave McCormick is one of several that could be the Senate majority-maker.

The race between three-term Democratic incumbent Pennsylvania Sen. Bob Casey and Republican challenger Dave McCormick is now rated a toss up — and the self-described “underdog” tells The Post he expects only more gains at the polls.