In a series of tweets, Schmidt slammed Trump and expressed disappointment he was leaving the party that once ended slavery.
Schmidt said it's become the party of Trump and is now "corrupt, indecent and immoral."
He called out the Trump administration's "zero-tolerance" policy for immigration, which has led to the separations of families during prosecution. He called the "worst abuse of humanity in our history," and said it's connected by the "same evil" that separated families during slavery and broke up Native American families.
"It is immoral and must be repudiated," he wrote. "Our country is in trouble. Our politics are badly broken."
Schmidt, a former aide to Arizona Sen. John McCain and political commentator, said he's spent much of his life working in Republican politics.
"I have always believed that both parties were two of the most important institutions to the advancement of human freedom and dignity in the history of the world," he tweeted. "Today the GOP has become a danger to our democracy and values."
Schmidt considers himself an independent voter and said he will now align himself with Democrats, whom he called the "only party left in America that stands for what is right and decent."
Other Republicans have recently condemned the party under Trump's leadership.
Former Hawaii Rep. Charles Djou announced in March he was leaving the party because of Trump's "hostile" immigration practices. Former Speaker John Boehner said earlier this month Trump had completely taken over the GOP.
29 years and nine months ago I registered to vote and became a member of The Republican Party which was founded in 1854 to oppose slavery and stand for the dignity of human life. Today I renounce my membership in the Republican Party. It is fully the party of Trump.- Steve Schmidt (@SteveSchmidtSES) June 20, 2018