Haley, who backed Romney in her state's first-in-the-South primary, is the youngest sitting governor and her husband will deploy to Afghanistan next year. She probably will have a strong message for military families, and for younger voters.
Martinez, who made history in her state and nationally when she was elected, could appeal to Hispanic women, a sizable demographic that broke for Obama four years ago. She can also address voters who feel securing the nation's Southern border is a top concern.
And Rice, an academic who served President George W. Bush as national security adviser and as secretary of state, could appeal to working women and those who put the United States' security as a top concern.
Some suggested she would be an excellent choice for Romney's running mate but Republican conservatives led a revolt, citing her support for abortion rights.
Romney, with limited foreign policy credentials, needs leading foreign policy figures like Rice to vouch for him.
Another prominent voice on foreign policy, McCain, will speak up for Romney.
The Senate veteran, who was a prisoner of war during Vietnam, remains among his party's most visible figures. His dislike for Romney has apparently faded since their primary fight four years ago.
"In these challenging times, America needs Mitt Romney in the White House," McCain said in a statement Republicans planned to release Monday. "The Republican National Convention in Tampa will help give us the momentum to get him there."
Another GOP rival from 2008, former Arkansas Gov. Huckabee, also will try to help Romney.
Huckabee's appeal among social conservatives has not shrunk and his backing is likely to help evangelicals who have been slow to warm to Romney and his Mormon faith.
Among tea party supporters, Romney will get a boost from Kasich. His home state of Ohio is a linchpin in Romney's strategy and no Republican has won the White House without carrying the perennial Midwestern battleground. No Democrat has won without winning Ohio since John F. Kennedy won the presidency in 1960.
Florida is another key state for both campaigns. Florida's Gov. Scott will address the convention, customary when the incumbent governor's party hosts the convention.
Democrats have started rolling out their convention schedule. Marking a first for Hispanics, San Antonio Mayor Julian Castro will deliver the high-profile, prime-time keynote address on the convention's opening night, Sept. 4, in Charlotte, N.C. First lady Michelle Obama will also address convention delegates that night.
Former President Bill Clinton and Elizabeth Warren, the party's popular Senate candidate in Massachusetts, will have prime speaking roles at the convention on Sept. 5.
Vice President Joe Biden and Obama will speak in prime time on Thursday, Sept. 6, the convention's final night.
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