A majority of independents voted for Democratic candidates on Tuesday, saying they were distressed by the Iraq war, disapproving of congressional leadership and concerned about the direction President Bush is leading America.

In a year in which both parties were playing to their bases, the center of the nation's electorate was nonetheless vital in this contentious midterm campaign as they cast ballots to decide which party would control the House and Senate.

According to interviews with voters as they left their polling places, about 4 in 10 overall said the economy, corruption and Iraq were extremely important factors in making their decisions. About 3 in 10 voters said terrorism, values and immigration were equally important.

Almost 6 in 10 voters disapprove of the war in Iraq, according to the exit polls, and about the same number believe the United States should withdraw some or all its troops. In voting for congressional candidates, nearly three-quarters of voters said corruption and government scandal were extremely important in their decisions on Tuesday.

While Bush was a central character in the race, particularly in the television advertisements and talking points of Democrats, only about one-third of those interviewed said they cast their votes out of specific opposition to the president. About the same number of voters said Bush did not play a part in their decision, while about 1 in 5 voters said they viewed their ballot as a way to support the White House.

Still, the president's sagging popularity provided an unavoidable theme for the campaign, and about 6 in 10 voters said they disapproved of how Bush was handling his job. A majority of voters declared that the nation was seriously on the wrong track, and about half of voters said the state of the economy was bad.

Nearly twice as many voters said national concerns outweighed local issues as they decided which candidates to support. That was precisely the sentiment Republicans had vigorously worked to avoid, trying to keep the fight for control of Congress as a series of individual elections rather than a national campaign.

The exit poll surveys were conducted nationwide by Edison Media Research and Mitofsky International for the National Election Pool comprised of the television networks and The Associated Press.