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KAI RYSSDAL: As the political race tightens this fall, so does the financial one. Campaign documents were filed with the Federal Elections Commission today. Senate Democrats had a healthy September, brought in $13.5 million. The GOP did slightly less than half that, about $5 million or so, but Republicans have more in the bank as we head into the last three weeks before Election Day. Marketplace's Scott Tong has this look at how parties know where to go to get the cash.

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SCOTT TONG: Not to creep you out or anything, but you do know you're being watched, right?

For years, Madison Avenue's known a lot about you: Where you shop, what kind of mortgage you have, if you prefer beer or wine, boxers or briefs . . .

Well politicians have joined the game. It's called data mining or microtargeting.

To get an idea how it works, I visited Democratic political strategist Ken Strasma. He's a door-locker 'cause this is stealthy stuff. Each party tries to keep its methods from the opposition.

Strasma takes me to a computer room.

KEN STRASMA: We're in what's called the server closet here, standing next to a big rack that holds our server. Which has data on about 150 million voters nationwide.

In other words, just about all of us.

STRASMA: 17,000 different data points that we know about each one of those voters.

Each of our files includes our name, address and party affiliation, if we have one. That's for starters.

STRASMA: But some of the new and more interesting stuff is, if you're a hunter and have a hunting license, that's available. If you subscribe to religious magazines, are you a cat owner, do you ride a motorcycle, do you drive an SUV, how many minutes do you commute to work each day.

From that, Strasma figures out what your habits tell him about your politics and then he assigns you a label. Here's an example, a 2004 campaign ad from an interest group.

[ Campaign ad: Howard Dean should take his tax-hiking, latte-drinking, sushi-eating, Volvo-driving, New York Times-reading . . . ]

Now in this case, the liberal tendencies may seem obvious, but it's not always the case.

Strasma says if you're a Republican, you tend to drink Coors or bourbon, drive a Mercury car and watch college football.

If you're a Democratic, you're statistically more into talk shows, brandy and theater.

Strasma feeds all this info to campaigns, which then make specific pitches to, say the theatergoers or the Coors drinkers.

So how do the politicians get all this consumer dirt on you and me?

Carol Darr at George Washington University says they buy it, from corporate America.

CAROL DARR: You should assume that the mass marketers know about any transactions with respect to which you have swiped a credit card.

Once you leave a digital footprint, it's out there. Give a website your name and email, it's for sale. Same thing with a sweepstakes or a raffle. Seems free.

LILLEY CONEY: There's no such thing as free any more.

Lilley Coney is with the Electronic Privacy Information Center.

CONEY: Even when you buy an item, and you want to get the warranty, filing out a warranty form and sending that in, it's a method of gathering information in order to feed into this new industry called data brokers. And they buy and sell all kinds of information about people.

Now it's just about impossible to opt out of the data gathering, but you can limit your exposure.

When you fill out a form, give as little info as possible. Say no at the grocery checkout when they ask you for your phone number. And uncheck that box on the website that says, would you like more information about . . .

Now if you all that, and the campaigns still call you, you can ask to be taken off their list. Legally they don't have to, but consultant Ken Strasma thinks they probably will.

STRASMA: A campaign wants to make a good impression with a voter and the last thing you want to do is to annoy them.

In the end, privacy advocates say the key is for you as a voter to know what the microtargeters are doing: They're tailoring a political ad to you, to push what they think are your buttons.

In Washington, I'm Scott Tong for Marketplace.