President Javier Milei scored a decisive political win Sunday, tightening his grip on Argentina’s Congress and securing a lifeline for his audacious free-market revolution backed by President Trump. With nearly 92% of votes counted, Milei’s Freedom Advances party won almost 41% of the national vote, putting it on track to more than double its representation in Congress. That means it should secure at least one-third of the seats in both chambers—the critical threshold that allows Milei to preserve his veto power and defend his sweeping decrees. The result, stronger than most polls had predicted, gives Milei fresh political momentum after...