More than 20,000 voters had cast their ballots as of 11 a.m., according to Frederick County Election Director Stuart Harvey.
Despite the rain, voters are still turning out, Harvey said. Harvey estimated that from 10 to 20 percent of registered voters in most precincts had already cast a ballot.
"This has the potential to be the biggest voter turnout in terms of percentage, of any midterm election I've worked on," he said.
Polls countywide close at 8 p.m. Voters in line by 8 p.m. get to vote.