WASHINGTON — A federal judge issued a nationwide injunction blocking a sweeping regulation set to qualify millions more Americans for overtime pay Dec. 1, delivering a blow to one of President Barack Obama’s signature workplace rules.

The Labor Department completed the rule in May as part of the Obama administration’s goal to restore and expand the middle class — either by raising incomes for workers in industries such as retail, food service and beyond or by giving them back personal time they’ve forgone while working extra hours unpaid.