In almost any election there are voters that are voting for reasons other than who will be senator. In many districts there are people who registered and are going to vote because they are for or against a school levy, or a zoning issue, or a wet or dry issue on the ballot. They may be voting for or against some country commissioner or mayor in some town.
Such people often do not even know who is running for the Senate, so when the pollster asks they are undecided.
Some may be ticked off at the Democrats, but unable to bring themselves to actually vote for a Republican. They tell pollsters they are undecided too.
The object of negative ads is to reduce support for a candidate. The object of negative ads is to turn a supporter into an undecided. In California where both candidates for governor have run very negative campaigns the undecideds are about 26 percent. Most of those will not vote on election day for either candidate. They will likely vote in other contests but leave the governors race blank.
A lot of that 10 percent are likely independents and Democrats who were moved into the undecided column by their reaction to the First Annual Wellstone Memorial and Beer Bash Pep Rally.
Minnesota seems split 3 ways with Jesse Ventura independents a third force. Ventura's comments about the pep rally favored Republicans. That swings some "independents" Republican. This fits with the fact that compared to Wellstone's independence, Mondale has always been a "made man".
Of the alleged 10% undecided, how many voted for Ventura last time? Remember that people can register to vote on election day in MN. The system encourages procrastinating to the last minute for everything.