In tonight’s New Hampshire encounter among Republican presidential candidates, sponsored by the Washington Post and Bloomberg, there will be the obvious things to watch for: Does Rick Perry finally turn in a decent debate performance? Can Perry and the rest of the candidates finally put Mitt Romney on the defensive by going after the former Massachusetts governor’s — take your pick — inconsistencies or position changes on a variety of issues, from guns to abortion?

But here’s another drama that matters: Can either Rick Santorum, the former Pennsylvania senator, or former House speaker Newt Gingrich finally break out to (1) push Herman Cain back in the polls, and (2) offer a real challenge to Perry for the role as the conservative alternative to Romney?

Cain’s breakthrough is by far the most surprising development of this year’s Republican contest. It can be read in part as a vote of no confidence in the GOP field, with many Republicans apparently attracted to the most “outsider” candidate in the field. Ron Paul has his loyal libertarian base, of course, but as an African American businessman, Cain is the newest new thing on offer. A certain proportion of the Republican primary electorate also loves tax plans that promise big income tax cuts. Cain’s 9-9-9 proposal is the sort of thing that plays well with those people who talk about “cap gains” and “marginal rates” the way baseball fans discuss OBPs and ERAs.

The question is: Does Cain have staying power? It’s the job of Gingrich and Santorum to test this. Both have turned in rather good debate performances so far, and Gingrich has seen some improvement in a few polls. Gingrich and Santorum need a two-fer in this debate: a continuing Perry sag, opening up conservative terrain for them to occupy; and breakthroughs against Cain to shake some of his current supporters loose. (Will they challenge the sales tax in his plan in New Hampshire, the state that proudly has neither a sales nor an income tax?) Eventually, of course, they would need to take on each other. But first, one of them has to get to the point of being in contention. This could be the Gingrich-or-Santorum moment. (And I notice I’m not alone in this view — Chris Cillizza is also thinking about Santorum and Gingrich. The lesson for all bloggers: Post fast, or Chris will get there first!)

The debate is also vital, for completely different reasons, to former Utah governor Jon Huntsman. It’s taking place in New Hampshire, the state where he is making his first (and, if he doesn’t do well, last) stand. Huntsman has to decide whom he is talking to in this debate. His best chance in New Hampshire lies with independents who can vote in the Republican primary. That means he should take a chance and try to separate himself from the generally right-wing tone that will be set by the rest of the candidates. And you wonder how his rivals will deal with him. All of Mitt Romney’s foes have an interest in Huntsman doing well in New Hampshire and siphoning votes away from Romney (or, better yet from their point of view, beating Romney in the one state that Romney has to win). What’s been missing so far for Huntsman is a clear strategy. We’ll see if one is discernable this time.

This entire analysis assumes that Rep. Michele Bachmann has had her day and that she will continue to drop back. Even more than Perry, she needs something good and big to happen in this debate. If she figures out how to make that happen, she’ll deserve whatever bounce she gets.