Microtargeting uses computers and mathematical models to take disparate bits of information about voters  the cars they own, the groups they belong to, the magazines they read  and analyze it in a way to predict how likely a person is to vote and what issues and values are most important to him. Often these analyses turn up surprising results; for instance, Democrats have taken advantage of the fact that many evangelical Christians are open to hearing a pro-environmental message.

The technique first came on the political scene in 2000 when Mr. Gage convinced Karl Rove, the Bush campaign’s chief strategist, that microtargeting, which was widely used by marketing companies in the business world, could be applied to politics and would to lead to a Bush victory. That victory helped spur the development of the Republican Party’s celebrated Voter Vault, which candidates can dip into to track down likely supporters and avoid wasting time on voters who cannot be persuaded.

For years, Republicans had the landscape to themselves. More recently, however, Democrats, along with such allies as trade unions and progressive groups, have poured millions of dollars into building two formidable databanks. One is managed by the Democratic National Committee and can be used by candidates up and down the ballot. The other is Catalist, a for-profit company headed by Harold M. Ickes, a Democratic political operative, that specializes in providing data for scores of liberal groups supporting the Democratic ticket as well as for the Obama campaign itself.

“Candidates and organizations like the Sierra Club and the A.F.L.-C.I.O. now have the time and money to start focusing beyond basic microtargeting and to understanding more advanced voter behavior,” said Vijay Ravindran, chief technology officer at Catalist, based in Washington. “There is a breadth and depth that we did not have before. We were just trying to crawl. There has been a concerted, deep and well-financed effort to catch up with and surpass what has been done on the other side.”

Microtargeting has become so widespread that it is now used by all House and Senate candidates, on both sides, in state legislative races and, in some cases, all the way down the ballot to local school board elections.