The Republican primary race ended for all practical purposes almost exactly three months ago, when Rick Santorum suspended his campaign and made Mitt Romney the presumptive presidential nominee. Since that point, only Newt Gingrich among the contenders has gone on the campaign trail for Romney, on several occasions, with the latest appearance yesterday in Florida. Ron Paul has only grudgingly conceded lately that he won’t win the nomination. Until now, Santorum has been relatively quiet, expressing support in interviews but not appearing on the campaign trail. That will change tomorrow, as ABC News reports:

This weekend former Pennsylvania Sen. Rick Santorum will hold his first formal campaign event for Mitt Romney since he became the presumptive Republican nominee, sources close to Santorum tell ABC News. Santorum, who emerged as one of Romney’s last opponents standing after a hard-fought primary season, will appear on Saturday at the opening of the Romney campaign’s Greensburg, Pennsylvania victory office. It’s familiar turf for Santorum who used to represent that area of southeastern Pennsylvania in Congress. … Saturday’s event represents another step in the peace-making process between Santorum and Romney, and it is also a sign that the Romney campaign believes Santorum can be a helpful surrogate in the country’s economically-struggling Rust Belt area. Santorum was asked to participate in the Greensburg office opening by the Romney campaign and Republican party officials, and “he was happy to accept the request,” according to a source familiar with the planning of the event.

This is a smart move for both men. Santorum drew a lot of support from Rust Belt Republicans with his economic policy and his middle-class approach. Romney needs to energize the Reagan Democrat voters in places like Ohio and Pennsylvania, as well as in Wisconsin, Iowa, and Michigan. Working-class voters may relate better to Santorum, while Santorum won’t steal Romney’s thunder. Plus, Santorum’s credibility on social conservatism will help assuage lingering doubts among that faction within the GOP about their nominee.

Santorum wants a career renaissance, and in order to succeed, he needs to show that he can be a team player. If Santorum can deliver Pennsylvania for Romney — a tough but not impossible task — he could gain back the rest of the electoral credibility he lost in 2006 after coming back nearly all the way in the 2012 primaries. A win in Pennsylvania would all but guarantee Romney the White House, but even a close loss would prove valuable in forcing Obama to spend suddenly-limited resources on defense in what had been a reliable Democratic state until 2010. Either way, Santorum’s star would rise.