Fox News and Google will host the next presidential debate tonight in Orlando. In addition to the usual cast of characters, former New Mexico governor Gary Johnson will be on the stage. (Swell, another eccentric candidate taking up time.)

The debate tonight is essentially about momentum. Texas Gov. Rick Perry, sliding somewhat in the polls, needs a turnaround debate in which he responds to nagging issues (Mitt Romney was on his case about Social Security yesterday), shows some command of the facts and policy, and demonstrates some stamina (he’s faded in both of the earlier debates in which he participated). The rap on him is that he’s very capable of delivering a one-liner, but when trapped in an extended discussion (e.g. on HPV vaccination, Social Security) he comes up short. That perception will no doubt induce some of his opponents to come back at him after his initial answer, even if the moderators fail to ask cogent follow-up questions. In past debates Perry has seemed under-prepared and taken aback by predictable questions. You can bet that his opponents will start pressing him on a whole range of issues. Perry will need to show he can withstand the heat.

Mitt Romney has had generally strong outings but must know Perry will come after him on health care and the comparative job-creation records of their two states. His challenge is to build on his momentum in the polls and in the debates, convincing wavering voters that he’s plenty conservative and much less risky in the general election. If Perry needs to show seriousness and maturity, Romney could use some fire-in-the-belly. If he’s going to defend RomneyCare, he needs to do it with passion and go after Perry’s record (25 percent of Texans lack health care). If he’s going to rely on his business expertise, he needs to show some righteous indignation when challenged on his Bain experience, making the case that conservatives are suppose to respect the free market — both successes and losses.

Rep. Michele Bachmann (R-Minn.) and Rick Santorum are heading in opposite directions. Bachmann has been losing ground in the polls and ensnaring herself in controversies since her win in the Ames straw poll. While she’s been on the decline, Santorum has begun to hit his stride. He’s turned in solid debate performances and remains a favorite of social conservatives. Can he, in effect, take her place as the tough conservative challenging the money-rich but policy-poor top dogs?

The rest of the field is becoming increasingly irrelevant. Rep. Ron Paul (R-Tex.) has been exposed (for those who hadn’t caught on before) as a crackpot. (A year from now he will be heading toward the end of his congressional career, thank goodness.) Jon Huntsman is still at 1 percent in the polls and shows no sign of developing a message that is attractive to the GOP electorate. (A year from now he’ll be an MSNBC commentator.) Herman Cain is amusing but not much more. (A year from now he’ll have jacked up his speaker’s fees and maybe have his own show on Fox.) Newt Gingrich has moments of insight and even wisdom, but they are too few and far between and are overshadowed by a flaky career. (A year from now he’ll have written two more books and be the customer of the month at Tiffany’s.) Gary Johnson might be able to get in a line or two, but he’s not likely to prove anything other than his ability to combine the fruitcake foreign policy views of Paul and the personal flightiness of Sarah Palin. (If Perry still believes what he wrote in his book, however, he and Johnson can discuss their common interest in allowing states to legalize pot.) A year from now, Johnson will go back to doing whatever he was doing before.

The real pressure should be on the moderators to ask probing questions, not ignore foreign policy, limit the time given to the fringe candidates and press the candidates to tell us about their specific policy views. Let’s hope they arrest the momentum of past debates, which have become increasingly unenlightening.