The Pennsylvania Supreme Court on Monday unanimously directed county election officials to stop defying the court’s guidance not to count mail ballots that were missing dates or had errors in the date field on the outer return envelope.

Last week, officials in several counties — including Bucks, Philadelphia and Montgomery — decided to count mail ballots with such errors on the outer envelope. They argued that the State Supreme Court’s pre-election guidance to throw out such ballots had not been decided on the merits, allowing them to interpret the election code and count the ballots.

The decision to count the ballots — about 400 in Bucks County, 600 in Philadelphia and an unspecified total in Montgomery — immediately became a source of controversy in the state’s contentious race for U.S. Senate. Dave McCormick, the Republican challenger, is leading Senator Bob Casey, the Democratic incumbent, by about 17,000 votes. The margin, less than half of a percentage point, has set off an automatic recount in Pennsylvania under state law.

The State Supreme Court’s decision tried to leave no room for interpretation for local election officials to continue to count the ballots.