The vote count in Florida’s Senate race keeps getting tighter. Gov. Rick Scott’s lead over the Democratic incumbent, Bill Nelson, is down to 15,000 votes, and it’s likely to narrow further as provisional and late overseas ballots are counted. As the initial count concludes, one issue will loom over the result: a substantial undervote in Broward County, the state’s most Democratic county, and the possibility that the ballot design, which might have made it harder to find the Senate choice, will ultimately cost the Democrats a Senate seat. An undervote is when a voter casts a ballot but doesn’t vote in one of the contests on the ballot. At the moment, there are a lot of undervotes in the Senate race in Broward. If Mr. Scott ultimately prevails by a margin of 10,000 votes or less, the undervotes in Broward County could be what cost Mr. Nelson the race. Broward County has reported about 25,000 fewer votes cast for Senate than for governor, a difference of about 3.7 percent. That means voters left their Senate choice blank, or the choice was not counted because of a tabulation error like an equipment problem. This is highly unusual, and there’s nothing like this discrepancy elsewhere in the state. Immediately across the county line in Miami-Dade County, about the same number of people voted in the Senate race and the governor’s race.