WASHINGTON—A former top Trump administration official targeted with a federal investigation by President Trump said he would fight back against what he called an unprecedented campaign by the government to punish dissent.Chris Krebs, one of Trump’s top cybersecurity officials in his first term, will resign from a leading security firm to more freely challenge the White House, he said. He joins a small but growing list of universities and law firms seeking to push back on Trump’s efforts to use the office of the president to attack critics and opponents.Â