Complex sentence: Bush was re-elected by a larger margin than in 2000, but won the smallest share ever of the popular vote for a reelected incumbent president; however he was the first Republican candidate since 1988 to win an outright majority.
Split into simpler sentences: Bush was re-elected by a larger margin than in 2000, but won the smallest share ever of the popular vote for a reelected incumbent president. However, he was the first Republican candidate since 1988 to win an outright majority.