Forget about Chris Christie and Newt Gingrich. The logical running mate for Donald Trump is former Sen. Rick Santorum of Pennsylvania.

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 Sens. Bob Corker of Tennessee and Joni Ernst of Iowa have been considered but have said in public they do not want the vice-presidential nomination.)

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

Political scientists say that vice-presidential nominees rarely swing their states toward their ticket. To the extent they matter, though, Santorum makes more sense than Gingrich or Christie. The 2012 presidential race was tighter in Pennsylvania than in Georgia or New Jersey. Santorum’s geographic background complements Trump’s message: Trump clearly wants white working-class voters in the Rust Belt to view him as their champion.

Like Gingrich but unlike Christie, Santorum would also bring Washington experience to the ticket. As a former member of the Senate Republican leadership, he could be a bridge between Trump and the party’s elected officials.

Santorum is best known as an outspoken social conservative. Social conservatives have their doubts about Trump, and Trump knows he needs to court them. They trust Santorum. They don’t have nearly the same level of trust in Christie.

Choosing Santorum would have its downsides, of course. He has often made impolitic comments, as when he promised that as president he would use the bully pulpit to denounce contraception. If Trump chose Santorum, Hillary Clinton would use it as more evidence that Trump is an extremist on social issues himself.

Since Trump so clearly does not care about social issues, though, it may prove difficult to make the charge stick. And Santorum’s gaffes are tame by Trumpian standards.

Santorum, Gingrich and Christie would all attack Clinton and defend Trump: In that respect they would all be roughly equally useful to Trump. All might be equally spoiled in his eyes because they have political identities independent from his; and Trump has said that he does not think it wise for bosses to hire people smarter than them.

But if Trump is looking for a conventional Republican politician to nominate for vice president, Santorum has an advantage over other potential candidates. Trump wants to run as an economic nationalist allied to social conservatives. Santorum is just the Republican running mate to solidify that brand.

E-mail Bloomberg View columnist Ramesh Ponnuru at rponnuru@bloomberg.net.

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