Six years after plunging Spain into its worst political crisis in decades, Catalonia’s separatist parties are in danger of losing their hold on power in the northeastern region after the pro-union Socialist Party scored a historic result in Sunday’s election.

The four pro-independence parties, led by the Together party of former regional president Carles Puigdemont, were set to get a total of 61 seats, according to a near-complete count of the ballots. That is short of the key figure of…