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Both campaigns want to claim momentum heading into the final days of the campaign. This is especially true in battleground states like Iowa, where enthusiasm and voter turnout can make all the difference.

It's a common political metaphor � momentum � but is it a good one?

Iowa State University physicist John Hauptman knows a thing or two about momentum: "Take an object. It's the mass of that object times it's velocity," Hauptman tells . "When a coach says his team has momentum, or a political figure says, 'My campaign has momentum,' they mean it's moving. And they mean it's moving in a certain direction � like up, for example, rather than down."

The best way to apply the metaphor to presidential politics is to think of volunteers and get-out-the-vote efforts as the "mass," and the speed with which the whole campaign is moving as the velocity, Hauptman says.

So is either candidate really on the upswing?

In a Web video called "" Romney claims he is. He looks back on the campaign and repeats several major themes, including his promise to "get this country back on track."

Romney's Iowa Strategist, David Kochel, , declaring: "It's clear the energy and momentum is ours."

The Obama team also points to volunteer support. "There is no doubt in my mind that the momentum is on our side," says Brad Anderson, state director for Obama's Iowa campaign.

More than half a million Iowans had already voted as of Thursday � a new record. That may bode well for Democrats, who have emphasized early voting; Republican officials say they didn't start a major push until the last week in September, when voting began in Iowa. Democrats outnumber Republicans both for the and for those returned.

But Democrats' early-voting advantage is narrowing as the election gets closer. In active-voter registration, Republicans in Iowa as of Thursday, albeit by only 1,400 votes in a state with more than 2 million voters. Independents still outnumber both Republicans and Democrats.

Physicist Hauptman says neither candidate can really claim momentum in Iowa, at least not with any scientific certainty.

"They've each lost their momentum. They're not moving. From one day to the next they may have positive momentum up or maybe negative momentum down," he says. "I would almost call this by another physics term if you'll pardon me: equilibrium, which means things are in balance."

Speaking of balance, both Obama and Romney planned stops in Dubuque, Iowa, on Saturday. Romney planned to return to Des Moines on Sunday. And, in a nostalgic move, Obama planned a final campaign rally in Des Moines on Monday evening � a nod, the campaign says, to "where it all began" during the campaign for the 2008 Iowa caucuses.

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