The recount underway in Pennsylvania’s U.S. Senate race marks the end of a chaotic post-election period that has become the latest example of how disputed election rules can expose weak points in a core function of American democracy.

The ruling is a major victory for Republican Senate candidate David McCormick, who holds a very narrow lead over Democratic Sen. Bob Casey ahead of a statewide recount.

Pennsylvania’s state Supreme Court is weighing in on a flashpoint amid ongoing vote counting in the U.S. Senate election between Democratic Sen.

The ruling comes after several Philly-area counties defied the court's previous guidance. The issue has come under close scrutiny as the race between Bob Casey and Dave McCormick undergoes a recount.

Pennsylvania's high court ordered state county election officials on Monday to not count "invalid" mail-in ballots in the U.S. Senate race between Democratic Sen. Bob Casey and his Republican challenger,