Cory Gardner in The Wall Street Journal

On Principles & Values: Giving up Energy Committee seat to run for Senate

Early last year, political handicappers viewed Udall, son of the late Rep. Mo Udall, as in good shape politically. But signs of Udall's political vulnerability grew. Like other Democrats, he drew blame for the troubled rollout of the health-care law. Criticism grew when his office questioned the number of notices the state said people received saying their policies were being canceled because they didn't comply with the standards of the new law. In late January Gardner began to reconsider.

The two leading GOP contenders in the race--Buck and Stephens--withdrew as the news broke about Gardner's decision to run. Still, Gardner expects a tough race. "We don't know if the [political] environment is better," he said. But "it certainly feels like it has turned the corner."