Leader of Italy’s most popular political party, Sen. Matteo Salvini, has been stripped of legal immunity and faces criminal trial for allegedly kidnapping migrants, offering him a risky platform to portray himself to voters as a persecuted defender of Italy against illegal immigration.

Italian prosecutors accuse Mr. Salvini of abusing his powers and kidnapping 131 asylum seekers in July, when as interior minister he refused to let the migrants disembark from an Italian Coast Guard ship that had rescued them.