Senator Cory Gardner, a first-term Republican, had trailed throughout the race as his state increasingly tilted toward Democrats.
Mitch McConnell has a tricky needle to thread.
Republicans see benefits for candidates struggling in conservative states, but Democrats say the vacancy sharpens their health care message and underscores Republican fealty to Trump.
As the campaign enters a critical period, the pandemic, their primary strategy and their presumptive nominee have given Democrats a shot at winning the chamber.
A challenger to Senator Mitch McConnell should finally emerge, and John Hickenlooper is trying to win his Senate primary in Colorado, among other races unfolding on Tuesday.
The former governor has faltered in a primary campaign for a seat that is essential for Democrats to capture if they hope to retake the Senate majority.
With jobless numbers rising, Republicans’ refusal to negotiate on another round of pandemic relief is proving difficult to sustain.
In contests in Kentucky, South Carolina, Kansas, Maine, Colorado and Arizona, Democratic candidates raised more money in the first quarter of 2020 than their Republican opponents.