State Rep. John Keyser, R-Morrison, resigned his legislative seat Monday and filed to run for the U.S. Senate seat held by Democrat Michael Bennet, The Colorado Statesman has learned from sources close to his campaign.

Keyser, an attorney and decorated Air Force reservist, will be joining a growing field of Republican candidates seeking to challenge Bennet, the lone Democratic incumbent considered vulnerable in this year’s Senate races.

Keyser also stepped down from his position at the international law firm Hogan Lovells Monday in order to devote full time to the Senate run, his campaign said in a statement.

“I’m running for the United States Senate to keep Colorado families safe from the growing threats we face across the world and to bring a new generation of economic opportunity to Coloradans after years of stagnation,” Keyser said in a statement. “Michael Bennet’s support for the disastrous Iran deal and his failure to stand up to his own party on national security issues has not only made Colorado less safe but has also emboldened America’s enemies.”

Backers of the first-term legislator say he’ll bring a focus on national security issues to the race as well as launch his campaign with the kind of fundraising commitments necessary to pose a credible challenge to Bennet, who helmed the Democratic Senatorial Campaign Committee in the last cycle and had more than $5.3 million reported cash on hand at the end of the last fundraising quarter.

Keyser’s background as a ground combat veteran in the war on terror — he holds the rank of major in the Air Force Reserve, served in Iraq and Afghanistan and was awarded a Bronze Star — could shine in a race Republicans say will be dominated by questions of national security, pointing to recent polling.

Following a visit to Washington, D.C., in early December, Keyser secured commitments of some $3 m

illion in soft money backing, a source familiar with the discussions told The Statesman. A source in the Washington foreign policy community had high praise for Keyser after meeting him at the Republican Jewish Coalition’s presidential forum luncheon, saying he was “blown away” by the Colorado Republican. “He schooled them on the ground intel of the region,” the source continued, saying Keyser has “absolute and complete foreign policy chops.”

Republicans have struggled to find a candidate to take on Bennet after U.S. Reps. Mike Coffman and Scott Tipton, as well as Attorney General Cynthia Coffman and 18th Judicial District Attorney George Brauchler decided against running despite heavy recruitment by state and national Republicans.

Conservative state Sen. Tim Neville, R-Littleton, announced his run in October and has been touring the state in the last week. Businessman and former congressional candidate Robert Blaha was scheduled to make his bid official on a Thursday morning radio appearance, after press time. Blaha had earlier paid for a TV ad set to air during the Denver Broncos’ playoff game Sunday and has also reserved $50,000 in advertising next week on Fox News.

Others in the primary include former Aurora Councilman Ryan Frazier, Jefferson County Commissioner Don Rosier, El Paso County Commissioner Darryl Glenn, former Parker Mayor Greg Lopez and Charlie Ehler of Fountain. El Paso County Commissioner Peggy Littleton is weighing a run.

Also this week, Parker resident Lily Tang Williams announced she was stepping down as state chair of the Libertarian Party and would be seeking the Libertarian nomination for the Senate seat. The Chinese immigrant got national attention last week for a photo that was widely shared on social media depicting her posing with a rifle in front of an American flag. “I will always stand with my AR, no matter what my President signs with his pen,” she wrote.

“There’s not a campaign yet,” Keyser told The Statesman in a telephone interview in early December, when he was deployed on a training mission in Florida to combat terrorist and transnational criminal networks in Central and South America. “But as I spend a few weeks serving in the military, I think now more than ever, our nation is at a crossroads and the threats we face are enormous. This is a pivotal time in our nation’s history.”

The liberal advocacy group ProgressNow Colorado filed a complaint Monday with the judge advocate general of the Air Force, charging that Keyser had violated military rules that forbid political activity or “behind-the-scenes” campaigning when he conducted the interview with the Statesman while deployed.

“We believe Keyser conducted political activity in uniform and is now lying about it,” said the organization’s executive director, Amy Runyon-Harms. “He should be investigated to determine the truth.”

A spokesman for the Keyser campaign roundly dismissed the charges.

“It’s very telling that Sen. Bennet’s special-interest friends are so worried that they would lie right out of the gate,” said campaign communications director Matt Connelly. “Clearly, Jon Keyser’s candidacy scares Sen. Bennet.”

Keyser’s resignation from the House District 25 seat in the western foothills of Jefferson County takes effect Jan. 25. Republican officials said a vacancy committee is scheduled for Jan. 23 to pick his replacement.

“The NRSC lost all of their top recruits to run for Senate in Colorado before they settled on Jon Keyser, and are now headed toward a crowded and divisive and damaging primary,” said Colorado Democratic Party spokesman Andrew Zucker in a statement. “No matter who emerges on their side, Michael Bennet will win in November because he’s worked across party lines to get things done for Colorado.”

— ernest@coloradostatesman.comThis story has been updated.