He becomes the first sitting Utah lawmaker to bolt his party since 2002, when Rep. Eric Hutchings, from Kearns, switched from Democrat to Republican.
Madsen is retiring this year after three terms representing Senate District 13 in Utah County and acknowledged that his move is "largely symbolic."
The move wasn't spurred by the GOP's nomination of Trump, he said — though he did cite concerns about the New York billionaire's inexperience. Madsen previously voted for Sen. John McCain, former Massachusetts Gov. Mitt Romney and President George W. Bush, he said, even though they weren't his preferred candidates.
Nor, Madsen said, was the switch driven by the defeat of his medical marijuana legislation in this year's session. He had "toyed" with becoming a Libertarian prior to running that bill, he said, and might have if he didn't think it might hurt the bill's chances.
He credits his move to a slow-boil change in the Republican Party — where he said "the scales tipped" away from those who espouse the strict civil liberties of Thomas Jefferson and toward those who favor the progressivism of Theodore Roosevelt — and his frustrations as a legislator in Utah, which he sees as "an island of progressive, restrictive law in the middle of the Intermountain West" and where he often found himself at odds with members of the Republican Party.
State GOP legislation has led to a population that is "less free," he said.
Madsen said a turning point came when he received calls from people in his former neighborhood complaining that the Utah County Sheriff's Office threatened to arrest them if they didn't evacuate their homes because of a growing wildfire.
His research found that every other state in the region allowed citizens to remain with their property. Once Utah law enforcement spoke in opposition, he said, it didn't matter. His was the only vote for the bill in committee.
When the issue of Medicaid expansion was raised in the Senate Republican Caucus, Madsen said he was one of a few who voiced opposition to expanding a "government entitlement program" that he said is "going bankrupt."
The current direction of the Republican Party sometimes "makes me want to cry," he said, "because I've invested a lot in that party."
Libertarian National Committee Chairman Nicholas Sarwark said Madsen's move is part of an "accelerating movement" that includes state legislators in Nevada and Nebraska and is the result of a widening rift in the GOP establishment.
"After the disaster that we saw in Cleveland, in which a party that was once storied has nominated a reality show star for president, where the convention chairman's cutting off the microphone of a sitting U.S. senator, it's broken," Sarwark said. "It's fundamentally broken and it's dying."