St. Paul, Minnesota - An Associated Press analysis of votes in the tight, still-to-be decided race for a U.S. Senate seat in Minnesota shows that most ballots lacking a recorded choice in the election were cast in counties won by Democrat Barack Obama.

    The finding could have implications for Republican Sen. Norm Coleman and Democrat Al Franken, who are headed for a recount separated by the thinnest of margins - a couple of hundred votes, or about 0.01 percent.

    About 25,000 ballots statewide carried votes for president but not for the Senate race. Although some voters might have intentionally bypassed the race, others might have mismarked their ballot, or optical scanning machines might have misread them.

    A recount due to begin Nov. 19 will use manual inspection to detect such ballots.

    Meanwhile, Coleman is using the state's open records law to ask Minnesota and all 87 counties for access to voting data and other records, questioning gains Franken has made since Election Day.

    Coleman campaign manager Cullen Sheehan complained of "statistically dubious and improbable shifts that are overwhelmingly accruing to the benefit of Al Franken."

    The Coleman campaign cited a 100-vote gain that Franken picked up from Mountain Iron, in St. Louis County, on Thursday night.

    Secretary of State Mark Ritchie said it was unfortunate that the Coleman campaign was questioning the integrity of the election, noting that adjustments are a normal part of the canvassing process.

    Paul Tynjala, St. Louis County's director of elections, said the change in results from Mountain Iron was because of a human call-in error on election night that incorrectly gave Franken 406 votes, instead of 506 votes.

    Three counties - Hennepin, Ramsey and St. Louis, which contain the population centers of Minneapolis, St. Paul and Duluth - account for 10,540 votes in the dropoff between the presidential race and the Senate race. Each saw Obama win with 63 percent or more of the vote.

    Larry Jacobs, a University of Minnesota political science professor, said the AP analysis of the dropoff between the two races creates a "zone of uncertainty" that could become a focal point for the campaigns and election officials - and agreed the numbers favor Franken.

    "These numbers present a roadmap for a Franken challenge," he said. "It suggests there are about 10,000 votes in the largest Democratic counties that are potentially going to tilt in Franken's direction."

    The ballots that showed a presidential vote but no Senate vote are called the "undervote." Statewide, more than 18,000 of those ballots came from counties won by Obama. About 6,100 were in counties won by Republican John McCain.

    Some areas of the state would appear to favor Coleman in a recount based on the dropoff, but most of those were smaller counties where the undervote was in the dozens. The largest of those pro-McCain counties was Anoka, in the suburban Twin Cities, where 1,189 ballots didn't choose a Senate candidate.

    Minnesota ballots are fed into optical scanners, which depend on voters filling in ovals to make their choice.

    Kim Brace, president of the consulting firm Election Data Services Inc., said there's no reason a ballot without a vote for a particular race would be rejected.

    "Usually they're set to kick back to the voter if there is an overvote," said Brace, who has been an expert witness in court cases stemming from disputed elections. "But in most instances they're not set to kick back to the voter if there is an undervote. After all, the public has a right to not vote for somebody for a particular office."

    Recount teams will look for whether stray or light marks on ballots signaled a voter's preference.

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    Associated Press writers Frederic J. Frommer in Washington and Martiga Lohn in St. Paul contributed to this report.