On Thursday morning, James Comey returns to a very familiar forum, and a very familiar conundrum. The former FBI director will testify before the Senate Intelligence Committee about his observations of the FBI's investigation into possible Russian influence on the 2016 presidential election, and especially whether any attempt to influence him by President Trump played a role in his abrupt termination. Comey has an opportunity here to publicly air any grievances over his firing, free of the professional requirements for discretion during open investigations.

So, will what he has to say make any bit of a difference? The problem for Comey is that both Democrats and Republicans have spent nearly a year attacking him for failing to exercise professional discretion when it mattered, and politicizing the FBI as a result. So whether his testimony will have any sway depends on whether he can overcome his massive credibility problem.

For a moment, it was unclear whether Comey would get to testify at all. Last week, White House counselor Kellyanne Conway suggested that Trump might invoke executive privilege to keep Comey from discussing a meeting between the two that took place in early February. This meeting in particular is of intense interest, as Comey will apparently tell Congress that Trump asked him if the FBI could drop its investigation into Michael Flynn after his resignation as national security adviser.

This unusual set of circumstances would have made executive privilege a problematic claim for Trump, at least politically, as it would have signaled that Trump thought there was something to hide. Trump himself has publicly discussed this conversation he and Comey had, and indeed ridiculed Comey on Twitter over it not long after firing him. Of course, Comey no longer works for the federal government either, so he has no incentive to refrain from public comment — and lots of incentive to stick it to Trump. It's this dynamic, as well as the possibility of official obstruction of justice, that has captured Congress' (and the public's) interest.

While the White House thought better of a privilege claim, Comey's testimony could be limited in other ways. Thanks to Deputy Attorney General Rod Rosenstein's decision to retain Robert Mueller as a special counsel on the Russia-influence probe, Congress has to tread lightly when it comes to the ongoing investigation. Mueller has already absorbed the Flynn matters as part of his investigation, which means that Comey might be a material witness. Fox News reporter Chad Pergram noted that committee chair Richard Burr (R-N.C.) plans to confer with Mueller first to "work out what's in-bounds/out-of-bounds in public" testimony, which suggests that Comey might not get to discuss everything that's on his mind.

At the same time, Comey might have to field some uncomfortable questions, especially if he now characterizes the February meeting with Trump and his own later firing as an attempt at obstruction of justice. After all, Comey testified to Congress a week before his termination that he had never been pressured to end an investigation for political purposes, almost three months after the Trump meeting took place. Why didn't he report it at the time, or when he first got fired, rather than waiting for the invitation from the committee to testify?

Unfortunately for Comey, he dissipated his credibility with both Republicans and Democrats on Capitol Hill during the FBI probe into Hillary Clinton. He outraged Democrats by publicly characterizing the conclusions of the investigation, and angered Republicans by not pursuing a prosecution over the secret email system and the serial mishandling of classified data. Having already done that once, Comey did it all over again just days before the election in a move which Democrats insist cost them the presidential election. And just before he was fired, Comey defended all of those actions, leading some Democrats in Congress to call for his termination.

The Trump administration may have (wisely) foregone the privilege claim, but they apparently plan to remind everyone of Comey's credibility issues. Rosenstein will testify before the Senate Intelligence Committee the day before Comey, in part to brief them on the decision to appoint Mueller. However, it doesn't take too much imagination to predict that Rosenstein will also review his memo to Trump about the need to act in some manner after Comey's usurpation of Department of Justice authority in both of his public actions in the Clinton case — and to remind some on Capitol Hill of their previous demands to terminate Comey for those transgressions, as well.

The Senate Judiciary Committee recently sent Comey a letter requesting answers to seven questions related to his meeting with Trump, along with any memos he wrote contemporaneously to record the contents of their conversation and any other such memos written after meetings with then-President Barack Obama and officials at the Department of Justice. According to online news site Circa, Comey politely declined to answer those questions on the basis of now being a "private citizen." If that's true, some on the Intelligence Committee might question why Comey wants to talk to them about Trump but not to the Judiciary Committee.

In other words, we can expect a lot of drama this week on Capitol Hill. Whether any of it moves the needle on these controversies remains to be seen, but given the damage done to Comey before this, don't expect to hear much that will stick.