Republican state Sen. Mark Baisley on Monday withdrew from the Colorado gubernatorial race and announced his candidacy for the U.S. Senate seat held by Democrat John Hickenlooper.

His departure leaves 20 Republicans running for the office held by term-limited Democratic Gov. Jared Polis.

“Colorado needs a U.S. Senator who will fight relentlessly for our Constitution, our economy, and our families,” Baisley said in a statement. “After traveling the state and listening to voters, it’s clear that Washington, not just Denver, is where this fight must be taken.”

Baisley joins four Republicans hoping to face Hickenlooper in November, including former state Rep. Janak Joshi, R-Colorado Springs, who made an unsuccessful bid for Congress in 2024; retired Marine Col. George Markert, a first-time candidate; Montrose County Commissioner Sean Pond, who declared his candidacy late Sunday; and Dathan Jones, the acting chair of the Alamosa County GOP.

Baisley told Colorado Politics that his move was prompted by Democratic state Sen. Julie Gonzales’ decision to challenge Hickenlooper in this year’s primary, suggesting that the incumbent — “the biggest name in politics in Colorado” — might be vulnerable. Baisley added that he believes there remain plenty of “good choices” in the crowded field of Republicans running for governor.

“I thought that I was a good option for Colorado for governor, but I also think that there are some other really great options for governor amongst the Republicans,” Baisley said. “So I don’t feel like I’m abandoning my state or putting them in a rough spot, that there’s not a good choice there. There’s several.”

Republican candidates for governor include state Sen. Barb Kirkmeyer, R-Brighton; state Rep. Scott Bottoms, R-Colorado Springs; Teller County Sheriff Jason Mikesell; Colorado Springs-based ministry leader Victor Marx; former congressional candidate Joshua Griffin; and conservative podcaster Joe Oltmann.

On Saturday, former Republican U.S. Rep. Greg Lopez, who was making his third run for governor, announced that he changed his voter registration to unaffiliated last week and would seek the office as an independent candidate.

Baisley said he doesn’t plan to make an endorsement in the gubernatorial primary but expects at least a few of his former rivals will support his Senate campaign.

Colorado’s Senate race isn’t considered competitive — nonpartisan election forecasters classify it as a safe Democratic seat — but Baisley said he anticipates his entry into the race could draw more national attention to the state, potentially including millions of dollars in outside spending.

Republicans haven’t won a statewide election in Colorado since 2016. Hickenlooper, a former two-term governor, unseated the last Republican to occupy one of the state’s Senate seats, Cory Gardner, in 2020.

Baisley, who runs Slipglass, a Littleton-based engineering software company, said his contacts in the tech industry prodded him to run for governor and will be on board with his bid for the national office. He also said he was encouraged to jump in the primary by members of the state’s congressional delegation and Colorado GOP Chair Brita Horn.

In an interview before his formal announcement, Baisley acknowledged that he has some ground to make up in fundraising, adding that his campaign team hopes to raise $25 million. That’s after bringing in just over $35,000 for his gubernatorial campaign through last year’s third quarter, according to the most recent available campaign finance reports.

Although Joshi and Markert reported receipts in the low six figures through the third quarter of 2025, Hickenlooper had raised nearly $6.5 million by the end of September and had more than $3.6 million on hand. Candidates face a Jan. 31 deadline to report fundraising activity through the end of the year.

“Obviously, it does take a ton of money,” Baisley said. “I have a new campaign team, and we are setting out to raise $25 million. It’s going to take the institutional money from Washington, but also, I think, the industry folks who will see me see me as good for them, including the aerospace industry.”

Baisley served two terms in the state House starting in 2018 before his election to represent Senate District 4 in the state Senate. Before running for office, Baisley served as vice chair of the Colorado Republican Party.

Markert’s campaign didn’t respond to a request for comment, but Joshi welcomed the new entrant to the primary field in a statement to Colorado Politics.

“This is good news for my campaign, because with more career politicians in the race, the more America First Republicans, conservatives and concerned independents will see that I am the only candidate who can beat a radical like Hickenlooper,” Joshi said in a text message.

A spokesman for Hickenlooper’s campaign said Baisley brings more of the same to a primary full of President Donald Trump’s supporters.

“Coloradans deserve leaders who will fight for lower costs and to protect their civil rights, not another MAGA Republican who will rubber-stamp Trump’s extreme attacks on Colorado and says being gay is a ‘rebellion against nature,'” Jess Cohen said in a text message.

At a Republican gubernatorial forum last fall, Baisley described his LGBTQ colleagues at the statehouse as being in “rebellion against the nature of God” and are using the state “as their kind of church to force their religious beliefs on the rest of us,” according to a video from the event.

Colorado Republicans and Democrats will designate candidates for the June primary ballot this spring.