Complex sentence: Rollins was a Whig for the first 20 years of his political career, but when that party broke up, he underwent a political transition, changing parties several times before becoming a Republican late in his life.
Split into simpler sentences: Rollins was a Whig for the first 20 years of his political career. When that party broke up, he began a political transition, changing parties several times before becoming a Republican late in his life.