Complex sentence: CAFE advocates assert most of the gains in fuel economy over the past 30 years can be attributed to the standard itself, and opponents assert economic forces are responsible for fuel economy gains, and higher fuel prices drove customers to seek more fuel-efficient vehicles.
Split into simpler sentences: CAFE advocates assert that most of the gains in fuel economy over the past 30 years can be attributed to the standard itself. Opponents assert that economic forces are responsible for fuel economy gains, and that higher fuel prices drove customers to seek more fuel-efficient vehicles.