As the 2020 elections approach, Colorado Sen. Cory Gardner is under fire. Democrats, of course, want him out. As one of only two sitting Republican senators representing a state Hillary Clinton won in 2016, Gardner is pegged by the Democratic National Committee as one of its top 2020 targets. And since 2017, left-wing Colorado activists have been beating their war drums against him. Armed with satirical, life-sized cardboard cutouts of the senator, and a ream of grievances, groups such as Progress Now Colorado and Indivisible Front Range Resistance, have held town hall meetings across the state, demanding Gardner’s head. The cries of dissent reached fever pitch when Gardner voted (with 50 other Republicans) not to allow more witnesses in the impeachment trial of President Trump. Days later, Gardner joined the majority of his Republican colleagues to acquit the president of the charges leveled against him by Democrats in the House of Representatives. “It’s time to unite and fight,” state Rep. Joe Salazar shouted Saturday during an anti-Gardner rally in Denver. “We vote, and we vote big. We are going to come out.” But uniting has proved difficult. Nine Democratic candidates are locked in a bitter primary battle for the Senate nomination, in large part because of former Gov. and erstwhile presidential candidate John Hickenlooper’s late entry into the race. Hickenlooper is still embroiled in a corporate ethics scandal from his time as governor. Many state...