As chairman of the House Energy and Commerce Committee, first elected to Congress in 1955 when Dwight D. Eisenhower was president, Mr. Dingell has probably fought more fights, intimidated more adversaries and pushed through more legislation than any other Democrat in the House.

But House Democratic leaders, hoping to pass an “energy independence” bill this month, have had to delay taking the measure to the floor for weeks. Speaker Nancy Pelosi and her allies want a hefty increase in fuel-economy requirements for cars, light trucks and S.U.V.’s, but they are finding that it is not easy to maneuver around Mr. Dingell, who wants a smaller increase that would be less painful for Detroit automakers.

The power struggle pits a towering committee chairman, long accustomed to running his own show, against the first female House speaker, who has her own ambitious agenda.