Gearing up for Wednesday’s big debate

>>> you have to assume both men have the facts down cold. one of them, after all, has been president for four years. the other has been running for longer than that. barack obama and mitt romney are both nearing the end of intense debate prep, and coaching sessions. and at this point, they're designed either to create or prevent that moment that we've all seen that can somehow change the race. and with millions of americans watching these debates like a kind of super bowl of american politics , the stakes are high as the two men face each other in denver, just two days from now. we begin tonight with nbc's andrea mitchell in our d.c. newsroom. andrea, good evening.

>> good evening, brian. it is the political super bowl , or perhaps the world series . mitt romney , behind in all battleground polls for the last three weeks, now has the most to gain from a game-changing night. and the most to lose if he doesn't turn it around. as the candidates head to debate camp , rolling up their sleeves, getting down to work, each claims to be the underdog.

>> governor romney, he's -- he's a good debater. i'm just okay.

>> he plays, barack obama , he plays them well, too. i hate to tell you.

>> reporter: mitt romney practicing with ohio senator rob portman looks like he's been getting a workout. the president sparring with john kerry , even chose a key suburb in nevada, a battleground state , for his practice session. neither candidate is known as a great debater. aides say the president doesn't like to talk in sound bites , and can be patronizing.

>> you're likeable enough, hillary.

>> thank you.

>> reporter: romney, say advisers, can come off as out of touch with average voters.

>> rick, i'll tell you what, 10,000 bucks? $10,000 bet?

>> reporter: in the 52 years since the debates were first televised the first rule is that appearances count. nixon should have shaved. al gore shouldn't have sighed. and the camera never blinks. challengers can benefit just by being on the stage with an incumbent president.

>> are you better off than you were four years ago?

>> reporter: it helps to have a well rehearsed one-liner.

>> governor, there you go again.

>> reporter: most memorably, lloyd bentsen 's takedown of dan quayle in 1988 .

>> jack kennedy was a friend of mine. senator, you're no jack kennedy .

>> reporter: but beware that deer in the head lights moment when a candidate forgets he's expected to be human.

>> governor, if kitty dukakis were raped and murdered, would you favor an irrevocable death penalty for the killer?

>> no, i don't, bernard.

>> i feel sorry for these candidates because there's a bunch of people sitting around a room right now telling them to do ten impossible things and then at the end saying be yourself.

>> reporter: and if you lose the first round you can recover at the next match with a well executed zinger.

>> i am not going to exploit for political purposes my opponent's youth and inexperience.

>> reporter: even a strong debate performance by john kerry wasn't enough to defeat george w. bush , but debates matter because it's the first chance to size up the candidates in unscripted moments one-on-one. brian?

>> andrea mitchell , who by the way will be our fact checker during our live coverage of the debate wednesday night here on nbc. andrea, thanks.