Complex sentence: In presidential elections when an incumbent is seeking reelection, the incumbent is traditionally his or her party's presumptive nominee: recent examples include Ronald Reagan in 1984, Bill Clinton in 1996, and George W. Bush in 2004.
Split into simpler sentences: In presidential elections when an incumbent is seeking reelection, the incumbent is traditionally his or her party's presumptive nominee. Recent examples include Ronald Reagan in 1984, Bill Clinton in 1996, and George W. Bush in 2004.