The most remarkable feature of that line-up is that none of them - with the exception of Pawlenty - ran in 2012 for the Republican nomination.

And only Rice and former Pennsylvania Sen. Rick Santorum are on the list of prominent Republicans invited to speak at the GOP convention.

Santorum is also planning his own events in Tampa to reach out to his staunch supporters separately from the Romney convention.  That amounts to the latest strain between the two former rivals in a season of angry interaction between Romney and Santorum.

It was Santorum who famously said Romney was "the worst Republican in the country to put up against Barack Obama." He also called Romney "the ultimate flip-flopper" and accused him of "lying to the American people."

There are other strains between Romney and his former rivals. 

The Romney campaign bus is rolling into the fall campaign without Rep. Michele Bachmann of Minnesota; Herman Cain, the radio talk show host; former House Speaker Newt Gingrich; Texas Gov.