Washington, D.C. ? The number of independent voters continues to rise, and that should give pause to Republicans trying to keep control of the House and Senate, an analysis of registration and turnout suggests.

About 17 percent of voters registered for next week’s election eschew major-party identification, according to a study released Thursday by American University’s Center for the Study of the American Electorate.

That’s up from about 14 percent in 2002, and the trend could help Democrats in their effort to gain 15 seats in the House and six in the Senate to achieve majorities, said the center’s director, Curtis Gans.

Gans said the numbers evoke statistical comparisons to 1962, 1982 and 1994, when voters turned against the incumbent parties out of “one form or another of unhappiness or fear.”

According to the study, 31 percent of registered voters identify themselves as Republicans, and 37 percent say they are Democrats.

Of the United States’ 138.8 million eligible voters, an estimated 42 to 47 percent are expected to vote Tuesday.