"Is it not un-American?" he asked. King also panned Trump's follow-up on Twitter, saying he acted like a "toddler that's been caught" when he defended himself with examples of Democrats citing opposition research.
"‘Everybody is bad,'" King said, referring to Trump's remarks. "'Don’t get mad at me because everybody is bad.'”
Trump, during comments revealed on Wednesday, told ABC anchor George Stephanopoulos that many lawmakers used opposition research from foreign countries.
On Twitter, he called out both House Intelligence Committee Chairman Adam Schiff, D-Calif., and Senate Intelligence Committee Vice Chairman Mark Warner, D-Va.
King pushed back on the idea that lawmakers use opposition research from foreign governments. "When a foreign government, especially a hostile foreign government, is the source and contacting you and offering you help -- that's not oppo research, that's a crime," King said.
King also defended his network, suggesting that it wasn't blowing the issue out of proportion. "If you're a supporter of the president out there, you're saying, 'there they go again' ... well, don't listen to us then, listen to him," he said before CNN played the clip of Trump's comments.