Colorado State Sen. Randy Baumgardner (R-Sulphur Springs) announced his retirement on Monday, months after an effort to expel him failed in the general assembly.

Colorado Senate Republicans told 9NEWS that Baumgardner’s retirement will be effective starting Jan. 21. The legislative session begins Jan. 10.

“It has been the honor of my life to serve the residents of both House District 57 and Senate District 8 over the last ten years,” Baumgardner wrote in his resignation letter. “I am humbled by the support I have received in my elections and even more so by those that have been positively affected by legislation I have worked on.”

A third-party investigation earlier this year found that Baumgardner “more likely than not” grabbed and slapped a legislative aide’s buttocks in 2016.

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A resolution to expel Baumgardner failed in April - 17-17 - when Republicans had the majority in the Senate. Last session, Democrats would have needed seven Republican votes to reach the two-thirds majority to expel. This year, with Democrats controlling the chamber, they would need only four.

“During my time under the Golden Dome, I’ve learned many lessons, with none being important as to always put your family first. My family’s resilience and love have given me the grace to continue forward under the most difficult circumstances, and in light of new opportunities, I must put them first in order to fulfill my obligations as a father and as a husband,” his letter continued.

Baumgardner’s replacement will be selected by a Republican vacancy committee, and his departure ensures there will be a fourth state senator this session who wasn’t selected by voters. Sen. Matt Jones (D-Louisville), Sen. John Kefalas, (D-Ft. Collins), and Sen. Daniel Kagan (D-Cherry Hills Village) also submitted their resignations.

Leaving after the session begins opens the possibility that their replacements could serve the remainder of the senators’ terms, and possibly be elected to serve two more terms after.

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