8 p.m. ET on, er, Bloomberg TV. Don’t get Bloomberg as part of your cable package? No problem: They’re livestreaming the debate right here. (If you don’t like that feed, there should be one somewhere on WaPo’s Post Politics page too.) The good news for Perry is that he doesn’t have to worry about immigration or Gardasil. They’ll spend the full two hours on the economy so he’ll have ample time to talk Texas and jobs. The bad news is that he’s one more catastrophic gaffe away from convincing everyone that he’s an irredeemably bad bet. If it happens tonight, wealthy donors who are leery of Romney’s squishiness will throw in the towel and back Mitt on electability grounds and grassroots conservatives who are leery of Cain’s inexperience will throw in the towel and back “Herb” as a charismatic “true conservative” who can galvanize the base. (No wonder Pawlenty’s now second-guessing himself for quitting.) In fact, if you believe lefty pollster PPP, it’s already come to this:

The other notable thing in these numbers is how much Rick Perry’s image has tanked in the last month. Only 23% of voters now have a favorable opinion of him to 57% with a negative one. That -34 favorability spread is worse than the -30 we found for Sarah Palin at 32/62 the last time we polled on her nationally, in August. He’s at an amazingly bad 16/62 with independents. Obama can only hope that Perry makes some sort of comeback in the next few months- it may be his only path to reelection.

If that’s true — and I find it hard to believe that it is — he’s already finished. Even if it’s not, I’m not sure a good debate tonight will be enough to restore him to co-frontrunner status with Romney. His team’s operating theory is that the surge for Cain right now is mainly an expression of doubt about Perry after his first few debates. Calm their fears with a solid performance, the theory goes, and those voters will come back, however grudgingly. I’m skeptical that that’s true given Cain’s personal appeal; I’m also skeptical that a major gaffe by Cain would hurt him as much as it would the other candidates. His whole argument, after all, is that D.C. needs a total outsider to shake things up. An outsider’s naturally going to have rough edges and an occasional knowledge gap on policy; that’s the proof that they’re an authentic populist, not some slick double-talking professional politician. As such, Cain’s margin of error is much wider than Perry’s, so even if he bungles something tonight it won’t take him out of the race. To really hurt himself he’d have to do something exceptionally stupid, like … accusing his supporters of being “heartless.”

Charlie Rose, the moderator, told Scarborough this morning that he’ll ask the field about Occupy Wall Street, so look out for that. I assume he’ll also break from the economic focus for at least a few minutes to put Perry on the spot about Jeffress and Mormonism since Romney and Christie went after him for it during this afternoon’s presser. (Perry’s son did some damage control this evening on his behalf before the debate.) Beyond that, much to my surprise, Cain said this afternoon that he’s going to go after Romney tonight instead of Perry. I thought he and Mitt might have some sort of informal alliance against Perry since he’s the chief obstacle for both of them — for Romney an obstacle to the nomination, and for Cain an obstacle to the title of tea-party champion — but maybe Cain’s now thinking seriously about the nomination too. He’s second in Iowa, just three points behind Romney, which is enough of a dogfight to encourage Mitt to visit the state again soon. I’d still be shocked if he hits him hard tonight but the mere possibility is reason enough to watch.

Here’s the handy Hot Air/Townhall Twitter widget to keep track of our updates. While we wait, read Robert Costa’s terrific tick-tock of how Bachmann’s campaign collapsed in Iowa. Or, if you prefer video, go watch Rush Limbaugh uttering a terrible, terrible truth on today’s show. Something needs to change, soon. Second look at Pawlenty?