Former labor union leader and independent candidate Dan Osborn has announced his bid for a Nebraska U.S. Senate seat, challenging Republican Sen. Pete Ricketts.

Billions of dollars in ads are raining down on voters across the Rust Belt, Rocky Mountains and American southwest as the two major political parties struggle for control of the U.S. Senate.

A former cereal plant worker running as an independent in deeply conservative Nebraska has emerged as a serious Senate challenger to two-term Republican Deb Fischer.