Complex sentence: Nelson was thought to be in danger of losing his seat in 2006, as it was thought his successor as governor, Mike Johanns, was almost certain to run against him; however, that speculation ended when Johanns was appointed U.S. Secretary of Agriculture.
Split into simpler sentences: Nelson was thought to be in danger of losing his seat in 2006, as it was thought his successor as governor, Mike Johanns, was almost certain to run against him. However, that speculation ended when Johanns was appointed U.S. Secretary of Agriculture.