For 2012 hopefuls, crucial days in Iowa

BRIAN WILLIAMS, anchor: These have been a strange few days in America , leaving a lot of folks feeling unsettled and demoralized after the US lost his credit rating and then lost 30 of its very finest in uniform overseas. Today we suffered more losses, and today we also got a glimpse of the anger in the country after all that we just witnessed in Washington . GOP candidates are in Iowa tonight, what they thought would be some straight up campaigning, some of them at the state fair prior to tonight's debate. The GOP smells opportunity in 2012 . But one man at a Mitt Romney rally had another idea today and it kind of set the tone for the rest of the day. We begin tonight with NBC 's Chuck Todd in Ames , Iowa . Chuck , good evening.

CHUCK TODD reporting: Good evening, Brian . Look , we're in the middle of the single most important 48 hour window in the presidential race today. On Saturday Rick Perry , the Texas governor , is going to make it official, get into the race. Tonight a very contentious debate is likely to take a place in Ames , and on Saturday, the straw poll could actually end some candidacies. Even before tonight's Republican debate in Iowa , GOP front-runner Mitt Romney found himself on the defensive.

Former Governor MITT ROMNEY: You know what? I get a chance -- I can let you speak in a moment.

Unidentified Man #1: You came here to the listen to the people, well, listen.

Gov. ROMNEY: No, don't -- no, no. I came here to speak. And you'll get to ask your question.

Man #1: We're going to...

Gov. ROMNEY: Hold on a second.

TODD: Visiting a state where he has spent very little time this year, Romney showed a rare burst of emotion when challenged on taxes by a group of Democratic activists.

Gov. ROMNEY: I'm not going to raise taxes, that's my answer. I'm not going to raise taxes. And if you want somebody that can raise taxes, you can vote for Barack Obama.

TODD: Moments later he was heckled again, this time over the issue of corporate taxes .

Gov. ROMNEY: One is we could raise taxes on people. That's not the way...

Unidentified Man #2: Corporations!

Unidentified Woman #1: Corporations!

Unidentified Man #3: Corporations.

Gov. ROMNEY: Corporations are people, my friend. We could raise taxes on -- of course they are. Everything corporations earn ultimately goes to the people. Where do you think it goes?

TODD: For months Romney has had the luxury of staying above the fray.

Governor RICK PERRY: All righty.

TODD: But with the formal entrance of Texas Governor Rick Perry into the race on Saturday, former aides say Romney will be forced to engage more.

Mr. MIKE MURPHY (NBC News Republican Analyst): When you're the front-runner, you don't like any strong opponent but you like clarity. And Rick Perry could bring some clarity to the race for Mitt as far as who his biggest opponent could be for the nomination.

TODD: Romney is not the only one feeling the heat. Former Minnesota Governor Tim Pawlenty has struggled to get out of the shadow of Minnesota Congresswoman Michele Bachmann . He has invested heavily in this weekend's straw poll and observers say it could be a make or break moment for him.

Professor DENNIS GOLDFORD (Drake University Police Science Department): He's got to do very well in order to stay alive politically.

TODD: Pawlenty is not alone. All of the announced candidates are crisscrossing the state this week, acting as if this might be their last few day on the trail. And to top it all off, Sarah Palin and her bus tour rolls into town tomorrow. Chuck Todd , NBC News, Des Moines .