President Obama is a huge fan of basketball but he doesn't seem to be much of a fan of boxing. If he was then he would recognize the "rope-a-dope" strategy used by Muhammad Ali. In his fight against George Foreman, Ali braced himself against the ropes and assumed a protective stance while Foreman rained blows down on him. Eventually Foreman tired himself out and the fresher Ali went on to win the match.
Over the Summer the Romney campaign has braced itself while the Obama campaign hit it with everything they had. Now, with the actual campaign starting, Romney has started to fight back against an opponent who has tired himself out.
Instead of actual blows, the Obama campaign has been burning through campaign cash trying to define Romney before he can define himself. The results have been questionable and the attacks have lowered Obama's likability rating. They have also blunted Obama's attacks. Obama's most effective attacks have been against Bain Capitol but those ads have been running so long that they are fading into the background noise. Unless the Obama people have a whole new line of attack they are stuck.
At the Republican convention the Republicans rolled out two new lines of attack. The first is to paint Obama as a faded leader who has not lived up to his promises. They have specific examples like the Janesville plant that Obama promised to save. They also have more general attacks. Ryan's best line was "College Graduates Should Not Have to Live Out Their 20s in Their Childhood Bedrooms, Staring Up at Fading Obama Posters". Romney's best line was "You know there's something wrong with the kind of job he's done as president when the best feeling you had was the day you voted for him." This is his version of Reagan's, "Are you better off today than you were four years ago?"
In 2008, Obama's acceptance speech was an event. It was too big for the convention hall so he moved it to a stadium complete with Greek columns. In 2012 Obama is just another politician trying for reelection during a stalled recovery. The contrast is inarguable and it gives Romney a great opening for attacking without being excessively negative.
The other line of attack is to show the real Romney. Most people know that he went on vacation with his dog strapped to the car roof (in a specially-built carrier). How many people know that in 1979 he befriended a dying 14-year-old from his church? Or that he closed the Bain Capitol offices so that the staff could help search for the missing daughter of a co-worker? Or that he twice pulled boaters out of a lake after their boats capsized? Stories like these show Romney as someone who goes out of his way to help people but they will be most effective close to the election before voters tire of hearing about them.
Obama has nothing similar in his background. If he did it he would have used it years ago.
To return to my boxing analogy, Obama is tiring while Romney is fresh and full of energy (campaign funds).