The short-list of potential Mitt Romney running mates appeared to get even shorter overnight, as Republicans confirmed another crop of convention speakers -- an assignment typically reserved for politicians not in contention for the party's No. 2 slot.

Add to the list of "not-its" two prominent names -- former Florida Gov. Jeb Bush and former GOP presidential candidate Rick Santorum. Both were confirmed overnight as convention speakers at the Tampa gathering later this month. Some vice presidential speculation had surrounded Bush, though the idea of Santorum and Romney teaming up on a ticket was always a long shot, considering their bitter rivalry during the closing weeks of the GOP presidential primary.

A former senior adviser for the former Pennsylvania senator confirmed to Fox News that Santorum has accepted the invitation to speak.

"He is certainly looking forward to sharing with America the reasons President Obama must be defeated, and inspiring conservatives all across the country to help elect Mitt Romney," the adviser said.

In addition to Bush and Santorum, a source confirmed to Fox News that Oklahoma Gov. Mary Fallin and Kentucky Sen. Rand Paul will also speak.

Seven major speakers were announced Sunday night, making it unlikely any of them are being considered. Those speakers are: South Carolina Gov. Nikki Haley, New Mexico Gov. Susana Martinez, Florida Gov. Rick Scott, former Secretary of State Condoleezza Rice, Arizona Sen. John McCain, former Arkansas Gov. Mike Huckabee and Ohio Gov. John Kasich.

Now, speculation can focus on who's conspicuously absent from that list as the Republican presidential candidate prepares to unveil his nominee likely before the Tampa convention, to be held Aug. 27-30.

Lately, the attention has been on those Republican luminaries who are expected to join Romney's upcoming bus tour. So far, the list of bus tour guests includes Florida Sen. Marco Rubio, Virginia Gov. Bob McDonnell and Ohio Sen. Rob Portman.

"The Romney Plan for a Stronger Middle Class" bus tour begins Saturday in Virginia, and then stops Sunday in North Carolina, Monday in Florida and Tuesday in Ohio.

Speculation has also followed Wisconsin Rep. Paul Ryan, former Minnesota Gov. Tim Pawlenty, New Jersey Gov. Chris Christie, New Hampshire Sen. Kelly Ayotte and Louisiana Gov. Bobby Jindal.

The Republicans have not yet announced their convention keynote speaker, which could easily come from that list as well.

Fox News' Mike Emanuel contributed to this report.