Tonight's debate looms large for both presidential campaigns. Barack Obama needs a comeback performance to erase the memory of a terrible first outing, which changed the momentum of the race. Mitt Romney would like to score a knockout blow and build on his momentum. Neither candidate will likely achieve their goals, given the dynamics of the format and its position within the general-election schedule.

First, the debate format doesn't lend itself to big battles between the candidates. While there will be room for digs against opponents, the town-hall format will force both Obama and Romney to focus their attention on the attendees asking the questions, likely voters provided by Gallup for the evening. That means being solicitous of their opinions and questions, even when they don't match up with the debate strategies of the campaigns. Each question will prompt a two-minute face-off between the two candidates in which they can go after each other's answers, but the back-and-forth between those interludes and the audience-participation rounds won't allow for much momentum for either side.

One wild card is the moderator, CNN's Candy Crowley. The rules agreed upon by both campaigns state that the moderator will not use the follow-up sessions to change topic or ask her own questions, but that's not how Crowley sees her role.

For Obama, the strategy is to look more presidential, less defensive, and more engaged than two weeks ago.

"Once the table is kind of set by the town-hall questioner," Crowley stated in interviews last week, "there is then time for me to say, 'Hey, wait a second, what about X, Y, Z?'" Not according to the Commission on Presidential Debates, as TIME's Mark Halperin pointed out — and not to either of the two presidential campaigns, either. Both filed letters of "concern" to the CPD, underscoring the trepidation both Romney and Obama have with any attempt to inject real spontaneity into a format that has much less of it than viewers might believe. The questions are pre-selected, not randomly generated on the spot, which explains why Crowley wants a larger role in the debate.

Furthermore, neither candidate does particularly well in town-hall-style forums. Most people will recall Romney's stiffness earlier in the primaries while conducting town-hall forums; the Los Angeles Times notes that more than one of Romney's missteps came during these interactions, including the statement that he likes to "fire people" as a way to show his enthusiasm for reducing government.

But Obama doesn't do particularly well in this format either. In 2010, when asked about ObamaCare and taxation in a friendly crowd, Obama gave a rambling, disjointed answer that lasted for 17 minutes. Most of Obama's gaffes, too, come from off-the-cuff moments. The nature of a town-hall format makes it difficult to prepare for all events with memorized answers, and those kind of answers sound too rehearsed for a supposedly extemporaneous forum.

Romney has worked at improving his performance in this format. Last week, while touring Ohio, Romney took questions from the audience at friendly rallies, and seems to have performed well enough — or at least avoided awkward mistakes. Obama, on the other hand, took a few days off the campaign trail for debate preparation without the benefit of any live audiences. Both seem to be focused on their own weak spots leading up to tonight's big event.

What about their strategies for the event itself? Romney's is straightforward: Continue pressing Obama on his record, especially on economics, as well as the bungled response to the Benghazi attacks. Obama did worst in the last debate when pushed into defending his record, although he should be better prepared tonight. Romney also needs to continue connecting on a personal level with the audience, both in the studio and outside of it, in order to continue countering Team Obama's demonization of him over the summer. Attack any straw-man arguments Obama erects, and look more presidential than the incumbent. 

For Obama, the strategy is to look more presidential, less defensive, and more engaged than two weeks ago. That's a tall order in this format, and may be a trap, too. Bob Woodward observed on Sunday that a suddenly ebullient Obama may create even more questions about his credibility. "He can't come on, in the next debate, and all of a sudden be a different person in a radical way," Woodward told Chris Wallace on Fox News Sunday. "People are going to say, 'Wait a minute, is it showmanship?'"

He also can't emulate his running mate's strategy of interrupting 82 times in 90 minutes, not without annoying the audience and looking desperate. Joe Biden can afford to look like a clown by mugging for the camera, but not the incumbent president — unless he wants to give away any potential advantage of gravitas that his office confers. Obama will need to cast himself as a steady hand, arguing a "stay the course" policy direction, which will only work if he's perceived as the steady hand on stage. He'll need to go after Romney's tax plan, and we can expect Obama to raise the specter of the "47 percent" statement that Romney made in a May fundraiser.

Will it make any difference? Unlike the VP debates, which have never impacted the trajectory of a presidential race, each presidential debate has the potential of being a gold mine or a minefield for both candidates. The reaction from the first presidential debate shows how much difference one event can make. In this case, though, the key debate has already taken place — and the key question was, could Mitt Romney be presidential enough to compete with Barack Obama on a stage? Romney won that debate by a wide margin two weeks ago, and that bell cannot be unrung easily. 

Even apart from the difficulties of this town-hall format, the specifics of this debate are likely to be lost by the first of November in favor of the first and last impressions of debates — and the final debate next week takes place on foreign policy, a topic on which the Obama administration finds itself under siege. In other words, this debate will matter, but it will probably matter least of the three. The incumbent has to hope that the challenger makes an unprecedented gaffe tonight to reverse momentum, but the plan will probably be to perform well enough to stop the panic among the base — and the perception among independents that Obama isn't up for the job.

Read more political coverage at The Week's 2012 Election Center.