In an introductory video, Stephens touts her time as Colorado House majority leader, during which she says she "stood up for middle-class families, passed balanced budgets and looked out for Colorado seniors."
She also knocks Udall as out of touch with Coloradans.
"Mark Udall has been in Congress for 16 years. During that time, he's grown out of touch with the struggles that families like ours face every day," he said.
And she pledges to "do everything in my power to fight against ObamaCare's negative impact on seniors, doctors, families and job creators until the day we're finally able to repeal it."
Stephens will formally announce her candidacy on Saturday.
Republicans believe that the closeness of the 2012 presidential and 2010 Senate races — a Democrat won both, but by single digits — means the state remains competitive for Republicans.
But their top-tier recruit, Rep. Cory Gardner (R-Colo.), opted out of the race, and a handful of other possible contenders decided against a run as well.
And Udall posted about $3.4 million cash on hand at the end of July, and has led all possible contenders in recent surveys of the state.
Colorado state Rep. launches challenge to Sen. Mark Udall
She joins state Sen. Randy Baumgardner, Weld County District Attorney Ken Buck and state Sen. Owen Hill vying for the chance to take on Sen. Mark Udall (D-Colo.) next November.