"In many cases involving predictions about human activity," writes Nate Silver in chapter 9 of The Signal and The Noise, "the very act of prediction can alter the way people behave." If you want to see this principle in action, look no further than the presidential race.

President Obama and Republican presidential candidate Mitt Romney face  moderator Bob Schieffer at Monday's foreign policy debate. ( Michael Reynolds/EPA/Pool)

The conceit of political reporting is that we reporters are telling you readers what the campaigns really think. In most cases, that's little more than a conceit. Campaign staffers aren't confused about who we are and what we do. So when we tell you what the campaigns say, We're really telling you what the campaigns want you to think they think.

The reality is that pretty much everything campaigns tell the press about how they're doing is strategy. It's strategy loosely bounded by reality -- it wouldn't be credible for senior Obama adviser David Plouffe to tell reporters that Texas was in play, for instance -- but it's strategy. And, going into the final stretch, the two campaigns appear to have precisely opposite strategies.

The Romney campaign is emphasizing momentum. Confidence. Even inevitability. You see it in their post-debate spin: "Mitt Romney did well enough that for the first time in six years, Romney folks emailed, 'We’re going to win.'" You see it in their electoral college spin: "Seriously, 305 electoral votes," an anonymous Romney adviser told Politico. You see it in their theory of the race -- that we're seeing a final break of unhappy independents toward the challenger, and that having permitted Romney to pass this commander-in-chief threshold, there's really nothing Obama can do to salvage the election.

The Obama campaign is emphasizing how tight it is. How hard they're going to have to fight this one out. How possible it is that the president might lose. Their latest fundraising e-mail, for instance, reads as desperate. It's supposedly from Obama himself, and the headline is, "Stick with me."  The first line is even graver: "I don't want to lose this election." Remember that this is coming from the campaign that won last night's debate and clearly leads in the electoral college. They could just as easily have written an e-mail entitled, "We're going to win this thing!"

What you're seeing here are two very different views of base psychology. The Romney campaign believes -- and polling confirms -- that Republicans are fired up to fire the president. They don't need to worry about voter enthusiasm. But they do worry about voter confidence. If Republicans don't believe they can win, they may not turn out to the polls. They don't like the former Massachusetts governor enough to turn out on his behalf. So as they see it, confidence is their friend: Every Republicans wants to say they helped turn Obama out of office.

The Obama campaign believes -- and polling confirms -- that Democrats aren't particularly fired up about the president. But they're very fired up by the idea that Romney might become president. For the Obama campaign, then, voter enthusiasm can be squelched by voter confidence: If Democrats don't think Romney can win, they may not be motivated to vote.

Until recently, this had Chicago pretty worried. Democrats have long seemed to believe they've got this election in the bag. That changed with the first debate (in part because the first debate really did make it likelier that Romney would win the election). So rather than comfort worried Democrats or strut about their debate win, the Obama campaign, in the final weeks of the election, is trying to scare its base, to persuade them that Republicans really could retake the White House. That gives their people a reason to go vote. And if their people have a reason to vote, the Obama campaign's ground game will do the rest.

The bottom line is that Boston fears scared Republicans won't vote and Chicago fears confident Democrats won't vote. And so, in this final stretch, Boston wants Republicans confident and Chicago wants Democrats scared. Keep that in mind as you read the spin.