Their scant personal relationship is in many ways the result of potential pathways to the presidency that had little to do with Washington. Obama spent just two years in the Senate before launching his 2008 White House bid. Romney spent most of his career in the private sector and has never worked in the nation's capital.
It's a marked contrast to many of the other political pairs that have faced off against each other for the presidency. Obama and his 2008 GOP rival John McCain, for example, had worked together in the Senate before facing each other in the general election.
Similarly, President George W. Bush and his 2004 Democratic challenger, Sen. John Kerry of Massachusetts, knew each other from work in Washington. Same with President Bill Clinton and then-Kansas Sen. Bob Dole, the Republican nominee in 1996.
Sara Taylor Fagen, who served as a political adviser to Bush, said Obama and Romney's lack of a personal relationship would likely be a mixed blessing on the debate stage.
"On the one hand, you've depersonalized it. You can say, 'I don't really know you, I'm totally comfortable saying whatever I have to," Fagen said. "On the other hand, familiarity is a really helpful thing. Even though you may be fierce opponents, it also gives you a comfort level."