After Comey's dismissal, news reports emerged that Trump asked Comey to end the probe into former national security adviser Michael Flynn during a February meeting in the Oval Office, the day after Flynn was sacked for misrepresenting his contacts with the Russian ambassador.
A member of the Senate intelligence committee says "we've seen no smoking gun at this point" regarding collusion between the Trump campaign and the Russian government during the 2016 USA presidential election. Under that constitutional principle, people in the executive branch of government can withhold information from Congress, courts and the public for certain reasons, according to a blog post by Mark Rozell, dean of the Schar School of Public Policy at George Mason University in Virginia and author of Executive Privilege: Presidential Power, Secrecy and Accountability.
The day after Trump allegedly requested Comey drop the Flynn investigation, reported CNN, White House Chief of Staff Reince Priebus reached out to Comey and other FBI officials asking them to push back on media reports about the Russian Federation investigation.
The decision was reported by the New York Times, which attributed information about the decision to unnamed senior officials with the Trump administration. Trump later tweeted: "James Comey better hope that there are no "tapes" of our conversations before he starts leaking to the press!"
Deputy Attorney General Rod Rosenstein who wrote the memo which the White House used to fire Comey will testify. Rosenstein wrote the memo that the White House used as the initial justification to fire Comey, and later called in a special counsel to take over the Russian Federation investigation.
Members of both houses of Congress last week declined to predict what Comey will say. A special counsel is looking into links between Trump's presidential campaign a year ago and Russians who sought to influence the election by hacking Democrats.
The White House appears to be considering raising the issue of executive privilege, but Trump may have a weak case for claiming that his conversations with Comey should be considered private, especially since the president himself has commented publicly about the circumstances surrounding Comey's May 9 firing.
This kind of an order or request can be interpreted by authorities as obstruction of justice which could have carried repercussions for Trump, especially as he sacked Comey thereafter.