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.

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,

The Pennsylvania Supreme Court on Monday ordered all 67 counties to comply with the commonwealth's existing Election Code and not include in tabulations, including the U.S. Senate recount, any mail-in ballots cast with dating errors on the outer return envelope.