Romney is currently the better-practiced of the two, having emerged victorious from a Republican primary season that featured nearly 20 debates. Graham says Romney is consistently solid, has great opening lines to questions, and has a firm grasp of the issues.
Romney's biggest weakness, according to some experts, is that he often comes across as fake. Graham says Romney's broad smile and "thank yous" following heated answers make it seems like "he's practicing his speeches," not debating his opponent.
Are debates about great politics or great theater?
Long stretches of presidential debates involve dry policy speeches, but it's usually a single gaffe or clever one-liner that comes to define a debate in the annals of national memory.
Reagan was already the oldest president in history in 1984, and when asked during a debate about whether age would be an issue for him, the 73-year-old famously replied: "I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience." Even Democratic challenger Walter Mondale, then age 56, had to laugh.
Sometimes body language matters more than words in a debate. In 1992, President George H. W. Bush took a glance at his watch while an audience member was asking a question - a move that made Bush, whose re-election hopes were rapidly slipping away, seem uninterested in the concerns of the public.
John McCain sparked controversy when he referred to Obama as "that one" during the second 2008 presidential debate. At a dinner attended by both senators several days later, Obama joked that his first name was Swahili for "that one," according to the New York Times.
Vice presidents and their counterparts have delivered just as many memorable lines as their bosses have over the years. Lloyd Bentsen's sharp "Senator, you're no Jack Kennedy" reprimand of Republican vice presidential candidate Dan Quayle in 1988 remains one of the all-time greats -- along with Perot running mate James Stockdale's "Who am I? Why am I here?" debate opener in 1992, which drew guffaws from the audience.
A bad enough gaffe can help derail your campaign long before the first primary votes are cast, as Republican Rick Perry showed in late 2011.
At a primary debate in Michigan, Perry became the first candidate in history to say "oops" during a debate after forgetting the name of the third government agency he'd pledged to cut.
When pressed for an answer, Perry said: "The third agency of government I would do away with, the Education ... uhh the Commerce, and, let's see. I can't. The third one, I can't. Sorry. Oops."
After the debate, Perry owned up to the gaffe as only a Texas governor could: "I'm sure glad I had my boots on because I sure stepped in it out there."