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.

Independent Dan Osborn announced another bid to represent Nebraska in the Senate on Tuesday, challenging GOP Sen. Pete Ricketts after his loss in an unexpectedly competitive race against a Republican incumbent last year.

Independent populist Dan Osborn, whose insurgent Senate campaign seriously threatened one of Nebraska’s Republicans from securing a third term last year, is now taking on GOP Sen. Pete Ricketts — one of the wealthiest members of Congress.