Complex sentence: Abramson was highly popular as mayor from 1986 to 1999 because of growth in the Louisville economy as the decline in urban population that began in the 1950s slowed greatly.
Split into simpler sentences: Abramson was highly popular as mayor from 1986 to 1999 because of growth in the Louisville economy grew. The decline in urban population that began in the 1950s also slowed greatly.