Romney is going to be perceived as the winner of the first debate barring any major gaffe. It will be the first time he has been elevated to that position and will appear right next to the president himself. That alone carries weight with some. As also mention the bar for Obama to clear will be higher than Romney's own. More is expected of the man in charge. The question we should be asking is he going to do something dramatic to shake the race up?
But as this election has proven once again, it is really hard to unseat an incumbent that is running, especially in the modern era. Once you are in, you have to really mess up for people to want to get rid of you. Let's take a look at the last 100 years:
Taft - Lost because his party was split by Roosevelt.
Hoover - Lost because he failed to ease the Depression and stood by his moral beliefs in volunteerism versus having the government run a deficit to help.
Carter - Was perceived as weak due to the hostage crisis. Oversaw the first energy crisis, and after calling it a malaise fired his whole cabinet. This does not inspire confidence. A recession and inflation hit right when he runs for reelection. Is undermined by his own party in a primary battle.
George H. W. Bush - Should have won reelection, but also had a recession on his reelection year. He also pissed off a lot of Republicans by breaking his pledge of no new taxes.
Out of the 18 presidents we have had since 1900, only 4 have been defeated in reelection attempts. And the main reason is because the economy tanked on their watch. It is surprising that Obama is doing so well today. I would attribute that mainly due to the public still blaming Bush for what happen. Another part of it is that we live in a more partisan time, and Obama has a higher floor of support than previous presidents. It also helps that, unlike Carter and George H. W. Bush, he isn't running against a talented politician. So it is never a good idea to bet against a sitting president winning reelection. People are willing to accept the devil they know versus the one they don't.