"The time comes when you have to be true to yourself and follow the dictates of your conscience," the 69-year-old former lawyer said at a press conference Tuesday. "For me, that time has come."
Specifying the reasons for his split, he pointed to the Trump administration's fiscal, foreign and environmental policies and the president's fomenting of what he called "hateful rhetoric and actions," all of which he said the U.S. will "soon pay a heavy price for."
McKean said the phenomenon in the White House is part of a broader shift in politics in which partisanship is favored over moderation, which made him "increasingly uncomfortable" with the Republican stance on a myriad of issues.
"If this is the new normal, I want no part of it," McKean said.
The Iowa House is now composed of 53 Republicans and 47 Democrats. The shift in balance follows a five-seat gain by Democrats during the 2018 midterm elections.
McKean's defection prompted a critical response from Republican Party of Iowa chairman Jeff Kaufmann, who said McKean is "about to feel the headwind" of Trump's support in his district.
"It's disappointing that he felt the need to deceive Iowans," Kaufmann said in a tweet. "If the people of District 58 can't trust him on something as simple and fundamental as what party he belongs to, how can they trust him on any issue."
Jessica Post, executive director of the Democratic Legislative Campaign Committee, said in a statement, "Representative McKean didn't leave the Republican Party. The Republican Party left him -- and their loss is our gain."