Michael Bennet

A year ago, Senator Michael Bennet looked like a lock to become Colorado’s next governor, with piles of campaign money, big-name supporters and a 30-percentage-point lead for the Democratic nomination.

That all changed in the 12 months leading to Tuesday’s Democratic primary, when President Trump took a battering ram to the Democratic state. Mr. Trump cut off federal money, dismantled a leading climate research center, removed the U.S. Space Command headquarters and vetoed a water pipeline for drought-stricken farmers.

Now, the Democratic race for governor has turned into a referendum on who will fight back, in pointed contrast to the accommodations of Colorado’s retiring Democratic governor, Jared Polis. And many Democrats are rallying behind Mr. Bennet’s rival, Phil Weiser, who is running as a scrappy counterpuncher who has sued the president 66 times as Colorado’s attorney general.

Mr. Bennet’s lead has evaporated, and Tuesday’s primary has turned into a dogfight that both campaigns say is now a tossup. The race will reveal a lot about which kind of leader Democrats frustrated with the political establishment want after two terms of Trumpism: a staid problem-solver or a revved-up fighter.

Despite growing voter disenchantment with Democrats, who have overseen spiraling housing prices, a rampant rise in homelessness and an economy that is now shedding jobs, whoever becomes the Democratic nominee is heavily favored to win.

Colorado has not elected a Republican governor in more than 20 years, and the Republicans vying to become governor include a state legislator who has warned of a pedophile ring in the State Capitol, and a pastor who claims he was forced to murder a man at age 7.