Announcing his switch to the Democrats, a Colorado state senator said Republican attacks on democracy were not the only “existential threat” posed by his former party.

“I have become increasingly worried about our planet and the climate crisis we are facing,” Kevin Priola said, in a letter posted to social media on Monday.

“The Republican party I joined decades ago created national parks, preserved federal lands and protected wildlife. President Nixon signed the legislation that created the Environmental Protection Agency (EPA). Today, my Republican colleagues would rather deny the existence of human-caused climate change than take action.”

Priola said such inaction would affect Coloradans already subject to “a near year-round wildfire season” and “a seemingly never-ending drought”.

Republican attempts to block “reasonable climate measures”, meant he could not stay silent, he said.

Priola was last elected in 2020, when he won a narrow race in the 25th senate district, in the Denver suburbs. He is due to serve until 2025.

Once a Republican bastion, Colorado has been trending left. Democrats already control the state senate, by 20-15 before Priola’s switch.

In his letter, Priola discussed dissatisfaction with the direction of the Republican party under Donald Trump, the former president who refuses to accept his defeat in 2020 while indicating he will run again in two years’ time.

“Like many Coloradans,” Priola wrote, “I watched the events on January 6 [2021] with horror. I felt that clearly this would be the last straw and that my party would now finally distance itself from Donald Trump and the political environment he created.

“Week after week and month after month, I waited for that response. It never came.”

Priola commended “brave and honorable” Republicans who stood against Trump after the Capitol attack, including the Utah senator Mitt Romney and Liz Cheney, the Wyoming congresswoman who last week lost her primary after taking a leading role on the House January 6 committee.

“Fear-mongering to raise money or motivate voters is nothing new,” Priola wrote, “but it has been taken to a dangerous and destabilising level.

“I cannot continue to be part of a political party that is OK with a violent attempt to overturn a free and fair election and continues to peddle claims that the 2020 election was stolen.”

Priola said he became a Republican thanks to the examples of Richard Nixon – the only president to resign in disgrace – and Ronald Reagan, but would now caucus with Democrats despite not sharing many of their positions.

“Our affiliations have become too tribal,” he wrote. “I’ve always been an independent thinker and … I don’t plan to change that. I do not believe either party has a monopoly on the truth.

“For instance, my pro-life position, school choice [support] and pro-second amendment stance often run counter to the Democratic party platform.”

But, he said, looking towards November elections in which Republicans on the national stage will seek to take back Congress, “we are in the midst of an election that will determine which party controls the [Colorado] senate chamber.

“Even if there continue to be issues that I disagree with the Democratic party on, there is too much at stake right now for Republicans to be in charge.”

Saying he had decided to “align with truth over conspiracy”, Priola concluded: “We need Democrats in charge because our planet and our democracy depend on it.”