The most powerful figure in Italy’s new populist government signed up on Friday with Steve Bannon, President Trump’s former chief strategist, to help bring about a continentwide populist takeover during European Parliamentary elections next spring.

Matteo Salvini, the Italian interior minister and the leader of the anti-immigrant party the League, has joined The Movement, a group founded by Mr. Bannon, the minister’s spokeswoman confirmed.

Mr. Salvini, whose popularity has grown with his power, amounts to Mr. Bannon’s first big get, lending legitimacy to his project and making it more likely that other Euroskeptic and populist politicians will join as well.

In an interview in Rome on Friday, Mr. Bannon said he and Mr. Salvini spoke for more than an hour in Mr. Salvini’s frescoed government offices about how his new Brussels-based organization would offer a “loose association” and physical space for right-wing populist leaders to meet and get to know one another and form alliances.