The tight margin in the Senate race between Sen. Bob Casey, D-Pennsylvania and Republican Sen.-elect Dave McCormick has triggered an automatic recount under Pennsylvania law.

An automatic recount has been triggered in the U.S. Senate race in Pennsylvania between Bob Casey and Dave McCormick. Here's how the recount will work.

The Pennsylvania Senate race between incumbent Bob Casey, left, and his Republican challenger, Dave McCormick, is headed for a recount. All counties must complete their recounts by Tuesday, Nov. 26. Pennsylvania's Senate race is going to a mandatory recount despite the Associated Press calling it last week for Republican challenger Dave McCormick.