Mitt Romney is behind in the polls, especially in important swing states but he is not so far behind that the race is a forgone conclusion. Both sides are running ads but these are unlikely to have much effect. The swing states have been buried with ads since last Spring. It is hard to believe that anyone who is still undecided will be moved by further advertizing.
That leaves the debates as Romney's best chance. There are three reasons why they will help him:
1) President Obama support is soft. Even the most positive polls have his support in the 50%-51% range. This does not leave him any margin for error. Undecideds often break for the challenger. If Romney can erode Obama's support by a couple percent and look good to the undecideds he can win.
2) This gives Romney national exposure. Romney's support has been highest when he gets coverage outside of campaign ads. Since the convention he has struggled to get any news coverage and when he does, it is often turned into a negative by hostile press. By contrast, Obama gained support after the killings in Libya because he was able to stand in front of the camera and look "presidential" (it also helped that he was able to disseminate a false story for a week about the basis for the attacks). The debates are Romney's last chance to reach the general public without going through the media filter. He doesn't have to "win" the debates. Simply standing on stage with Obama will give Romney stature. It also will help Romney that Obama is at his worst when speaking without a prepared speech on a teleprompter. Romney does not need a zinger to win. He just needs to look like a viable choice to be president.
3) Obama cannot avoid his record in the debates. He will try to frame it in the best possible light but Romney has multiple openings - yes, 4 million jobs were created but the workforce today is smaller than when Obama took office. Yes, there is a recovery but it is so weak that the Fed promised to keep pushing stimulus money for as long as it takes (QE3).
Both Reagan and Clinton saw gains from the debates. The same forces are at work here and will give Romney his make or break moment(s).