ROME — Italian Deputy Prime Minister Matteo Salvini faces a battle to save his far-right League party from electoral oblivion. The party’s internal crisis exploded into public view last week after Salvini’s maverick deputy, Roberto Vannacci, an ex-general and defender of fascist dictator Benito Mussolini, threatened to form a splinter party to the right of the League called National Future. Salvini seeks to play down the split with his No. 2, but Vannacci’s move revealed starkly how the League —