CINCINNATI - Mitt Romney's five-day, six-state bus tour is shaping out to be a series of auditions for potential vice presidential candidates.

Romney will kick off the tour Friday in New Hampshire, where Sen. Kelly Ayotte will join him on the stump. Ayotte campaigned for Romney frequently leading up to the primary, and has made two appearances with him since, most recently in mid-May.

Romney will head Saturday to the battleground state of Pennsylvania, which is the home state of former Sen. Rick Santorum, who endorsed Romney after abandoning his own bid for the White House. But a Santorum aide says the former presidential candidate was not invited to campaign with Romney. Santorum has not campaigned for or with Romney since endorsing him in early May.

Romney's bus Sunday moves into Ohio, where Sen. Rob Portman, whom many consider to be one of Romney's top choices for a running mate, will spend the entire day with the candidate, including a Father's Day pancake breakfast. Later in the day, Romney will be joined by House Speaker John Boehner for burgers, the first joint appearance by the two.

Monday finds Romney and his bus in Wisconsin, where he will campaign at a fabric manufacturer in Janesville, the hometown of Rep. Paul Ryan. Ryan will appear with Romney at this event. Ryan spent a four-day stretch with Romney on the trail in early April leading up to the Badger State's primary.

Sen. John Thune of South Dakota, who stumped for Romney during the primary, has no plans to jump on Romney's bus as of now, according to Republican sources. Nor does Florida Sen. Marco Rubio. Aides to Former Minnesota Gov. Tim Pawlenty declined to answer questions about his appearance on the bus tour.

Virginia Gov. Bob McDonnell is traveling in Europe for the duration of the Romney bus tour, ruling out a cameo on the tour.

ABC News' Shushannah Walshe, Michael Falcone, Gregory Simmons, Matthew Jaffe, Sunlen Miller and John Parkinson contributed.