Incumbent U.S. Sen. Bob Casey is not ready to give up his seat just yet, even as the margin of votes show the three-term senator trailing behind newcomer GOP candidate David McCormick in his re-election bid.

Democratic incumbent Sen. Bob Casey Jr. faces a well-financed challenge from Republican Dave McCormick in Pennsylvania’s nationally-watched U.S. Senate race. While Casey held a lead for much of the campaign, the race tightened in the final days ...