Before Trump entered the race, Santorum was the only Republican presidential candidate who argued that low-skilled immigration reduces wages for American workers. Trump rails against the North American Free Trade Agreement. Santorum voted against it. Trump frets about the fate of American manufacturing. In previous presidential campaigns Santorum stood out among Republicans by advocating government support for manufacturers. Trump says he wants the Republicans to be “a worker’s party.” Santorum wrote the book on it, literally: His “Blue-Collar Conservatives” was reportedly an influence on Trump’s campaign.

Speculation about Trump’s veep choice has centered on Christie and Gingrich, but Santorum has also been reported as a possibility. (Other politicians, such as Senators Bob Corker and Joni Ernst, have been considered but have said in public they do not want the vice-presidential nomination.)

Christie and Gingrich are not nearly as close to Trump on immigration and trade policy as Santorum is. They would either have to renounce their records on those issues, making themselves look insincere, or undercut Trump’s message by noting their disagreement with it.