The Democrats' request was prompted by reporting by Jane Mayer in the New Yorker that Trump ordered his chief economic adviser, Gary Cohn, to force the Justice Department to sue to stop the merger. According to Mayer's reporting, "Trump called Cohn into the Oval Office along with John Kelly, who had just become the chief of staff, and said in exasperation to Kelly, 'I've been telling Cohn to get this lawsuit filed and nothing’s happened! I've mentioned it fifty times. And nothing's happened. I want to make sure it's filed. I want that deal blocked!'" Kelly reportedly told Cohn, after that meeting with Trump, "Don't you fucking dare call the Justice Department. We are not going to do business that way."
Behind the order was Trump's animosity toward CNN and its parent company, Time Warner. Nadler told CNN after the revelation that if the report bears out, Trump’s action will be considered a threat to the free press, and while it was too soon to determine if it was an "impeachable act," he said it's "certainly an abuse of power."
Trump's refusal to answer is another abuse of power. If the Justice Department decided to intervene in the merger in order to harm CNN at the behest of Trump, that's a direct attack on the First Amendment, and on the free press.