Kucinich's loss in the newly redrawn 9th Congressional District came as 10 Super Tuesday primaries and caucuses among Republican presidential candidates dominated news coverage.
Kaptur won the primary by more than 20 points -- 60 percent to Kucinich's 36 percent, with 85 percent of votes tallied as of early Wednesday, the Washington Post reported.
Kaptur, a 15-term House veteran, is a member of the powerful Appropriations Committee. Kucinich, a former Cleveland mayor, has been a member of the House since 1996.
Kucinich's political career, which he began as a member of Cleveland's city council in 1969 and included two attempts for the White House, is now in doubt, several media outlets reported.