Complex sentence: Heinrich originally planned to challenge five-term Republican incumbent U.S. Representative Heather Wilson, of New Mexico's 1st congressional district, but Wilson retired to run for the U.S. Senate seat vacated by retiring Republican Pete Domenici.
Split into simpler sentences: Heinrich originally planned to challenge five-term Republican incumbent U.S. Congresswoman Heather Wilson, of New Mexico's 1st congressional district. However, Wilson decided to retire in order to run for the U.S. Senate vacated by retiring Republican U.S. Senator Pete Domenici.