President Trump threatened Wednesday not to participate in presidential debates sponsored by “fake news networks.” Riffing off a decision by the Democratic National Committee that Fox News is ineligible to host 2020 presidential primary debates because of pro-Republican bias, Mr. Trump said he’d reciprocate. “Democrats just blocked @FoxNews from holding a debate. Good, then I think I’ll do the same thing with the Fake News Networks and the Radical Left Democrats in the General Election debates!” he wrote.