His entry is certain to upend the nascent Democratic Senate primary, which already includes 11 candidates. Early polls have shown Hickenlooper outpacing Gardner in a hypothetical matchup. However, he is not expected to clear the Democratic field. Several Senate candidates pledged to stay in last week after Hickenlooper dropped out of the presidential race.

Partisan control of the U.S. Senate hangs on a handful of key races, including Colorado’s. Gardner is considered one of the most vulnerable Senate Republicans up for re-election in 2020, and national Democrats, who want to leave nothing to chance, leaned heavily on Hickenlooper to run against him.

By entering the Senate primary, Hickenlooper fulfills their wishes. However, his candidacy could dash the dreams of Colorado progressives who had hoped the seat would be filled by a woman or person of color — adding diversity to Colorado’s Washington delegation.