Romney's last chance to overturn Obama's small lead was during the conventions. But Obama came out with an increased lead (from about 1.5 to about 3%) and Romney has endured a terrible last couple of weeks. As the statistician Harry Enten explains, "There hasn't been a single candidate to come back after trailing by 3 points this late in the campaign in the past 60 years."
Furthermore, the latest polls are showing Obama with even bigger leads in the swing states, including Ohio and Florida. We also know that Obama's on-the-ground operation is far superior to Romney's, that he has at least as much money left to spend, and that for all the Democrats' concerns about the tsunami of outside cash being spent by rich Romney supporters, it turns out that most billionaire donors are too stupid for their money to make any difference.
Until recently, I've said what everyone else has been saying: Obama's biggest vulnerability is the slow economy, and it might still be his downfall. But now I think it's too late for Romney to take advantage there. He's had his shot. The weak economy is already baked into the current polls. I agree with Marc Ambinder, who thinks that "voters have already conducted their referendum on the Obama economy, and made their conclusions...If they are persuadable, they are persuadable on other issues, issues that Romney isn't going to find much traction with."
People will talk a lot about the debates being Romney's last chance to turn things around. It's true that Romney will probably be very good, because he's a highly capable debater who got a lot of practice in over the last year. But Obama is too solid a performer to quail, and anyway debates, though fun to obsess over if you're that way inclined, never really change anything.
Obama must be a hugely frustrating candidate to run against. He's so steady, so consistent, so dull, and so hard to knock off-balance. He also manages to sound nice and reasonable while heading an absolutely ruthless operation that exceeds even the 2004 Bush campaign under Karl Rove in its remorseless focus on winning. Romney is flailing around now, inventing new strategies on the run and throwing tactics at the wall in the hope that something, anything will stick. In other words, he is into John McCain territory.