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ABC News' Michael Falcone and Shushannah Walshe report:

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 southwestern Pennsylvania in Congress.

Up until now the former rival has not been in close touch with the Romney campaign. Santorum had said he was willing to campaign for the former Massachusetts governor, but since the meeting between the erstwhile foes in May there has been limited contact between the two camps, Santorum sources said.

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.

But ever since he ended his own presidential bid in April, Santorum hasn't always sound like the most enthusiastic Romney backer. The candidate who in March called Romney the "the worst Republican in the country to put up against Barack Obama" was asked in an interview on CNN last month if he now "trusts" the presumptive GOP nominee.

"Well, I trust him more than I do Barack Obama," Santorum told CNN's John King. "This election is about a choice."

Santorum's campaign appearance for Romney comes on the heels of a two-day tour of Iowa this week, which invited speculation about whether the former senator was going back to the state that handed him a belated win in the presidential caucuses not only to say "thank you" to his supporters there but also to lay the groundwork for a 2016 presidential run.

Last month, Santorum also launched a new independent group called "Patriot Voices," which he formed to promote the conservative causes that were the hallmarks of his campaign - from stem cell research to pro-life issues to foreign policy. He has been backing candidates and sending out issue alerts as well as fundraising e mails to his supporters, much like he did as a candidate.