Curtis Gans, director of the independent Committee for the Study of the American Electorate, estimated Tuesday's turnout at 77 million, more than 39 percent of voting-age citizens.
In the 1998 elections, the figure was 37.6 percent — the lowest midterm turnout since 1942.
Twenty-eight states had higher turnouts this year and 22 states and the District of Columbia had lower turnouts.
Gans attributed the higher turnout to the tight races and the parties' mobilization efforts. Democrats had hoped to build on their success in getting out the vote in the last election, while the GOP had tried to emulate the success of Democrats and organized labor.
In some cases, President Bush's personal popularity and his campaigning may have been factors, Gans said Wednesday.
"The Republicans nationalized the campaign and the Democrats did not," he said.
Gans said Florida had a record-high turnout, at least 6 percent higher than in 1998, because of Bill McBride's spirited, but unsuccessful, challenge to Republican Gov. Jeb Bush.
A close Senate race in New Hampshire drove turnout to more than 48 percent, about 12 percent higher than in 1998.
Turnout rose 6 percentage points in Missouri because of another close contest for the Senate and 5 percentage points in Massachusetts because of a hard-fought governor's race, he said.
More voters also went to the polls in Tennessee, Oklahoma and Delaware, but that may be because the last midterm election had no major issues or contests, he said.
Written By LESLIE MILLER