Julian Zelizer , a history and public affairs professor at Princeton University and a New America fellow, is the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society. " He's co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own.

The start of this testimony, which will be public, comes at a critical moment for Donald Trump's presidency. A dark cloud has loomed over this administration since the day that he was inaugurated. There has been a continued and steady flow of revelations about how people in the Trump orbit had some kind of connection or interaction with Russian officials at the same time that the US intelligence agencies have claimed there was a concerted effort to intervene in the election to sway the outcome toward the GOP.

Trump's supporters have dismissed the entire issue as "fake news," blaming Democrats and a biased media for drumming up suspicion about the new president simply because the party is still bitter about Hillary Clinton's defeat. There are some observers who have also warned that this all smacks of a new McCarthyism, a reference to the anti-communist hysteria of the 1950s, with liberals raising suspicions based on the most tenuous of connections and communication that is actually normal.

But for many others, this is an incredibly serious matter with huge implications for this administration. At one level, the investigations could reveal that the election was illegitimate. The question that a large number of Americans have is whether Donald Trump and his associates coordinated with the Russians to defeat Hillary Clinton.

At another level, if there is something there, the investigations could reveal that the Russians and Team Trump have connections that are too cozy, either because of financial ties, dirty laundry, or some other mutual need that will keep both parties working closely together.

At this point, President Trump doesn't have any credibility on this issue. We have now seen him and many members of the administration stretch the truth or outright lie when questioned about these connections, while his false claims about President Obama wiretapping his phone have exposed that his words can't be taken seriously.

There is pressure on everyone to step up to this challenge. For Comey, he needs to explain to the public what his agency has been working on, what they have found, and why they remained silent about this during the election even as he sent his now-famous letter in late October about looking into Hillary Clinton's email. If the FBI has evidence that there was some kind of collusion between the Trump campaign and the Russians, or that now the administration is taking a favorable stand toward the Russians because of incentives that have nothing to do with the public interest, this is his time to come clean.

The public deserves to know what the FBI has been finding and, just as important, what it has been looking into. The purpose of shifting this investigation into the congressional realm is because there needs to be a public accounting of what took place.

Although public hearings are often just stages for grandstanding, there is a long history, like the Watergate Committee and the Iran Contra Committee, of using these events to allow the public to learn about what happened behind the scenes in an administration. The time has come for Comey to explain what's been going on before television cameras and with reporters in the room. If the FBI is not finding anything that is serious, Americans need to know that as well.

Republicans in Congress can't just stand by their man. This investigation is a serious test of the credibility of the GOP. There are many Democrats who have been calling for the appointment of a special prosecutor or at least the creation of a select congressional committee and who lack confidence that Republicans on Capitol Hill will conduct a serious investigation into a Republican president.

In other words, they fear that the power of partisanship will trump any efforts to open up a real Watergate-style investigation into what occurred during the fall of 2016. Although the Republican-led committees are opening inquiries, the concern is that these will be watered-down efforts that end up shielding the administration. These concerns were fueled by revelations that Republicans on the intelligence committees worked with administration officials to refute charges that emerged in the media about this scandal.

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At the forefront of the GOP opposition have been Senators John McCain and Lindsey Graham, both of whom are hawks on the Russia issue and have little love for the new President. They have been openly dismayed as the President accused former President Obama of illegally "wiretapping" him at Trump Tower, and they have remained extremely interested in all of the evidence that has emerged about Russia's role in the 2016 campaign.

The credibility of the GOP hinges on its willingness to take this responsibility seriously. Lacking that, the party's ability to move forward with key items on its legislative agenda, including replacing the Affordable Care Act, will suffer. The credibility of this administration also depends on Congress proving to voters that it will uncover every piece of evidence that exists and ask tough questions of administration officials to make sure that there is no fire behind the smoke.

This is also a chance for President Trump to clear the air. At this point there are many people raising Senator Howard Baker's famous Watergate question: What did the President know and when did he know it? Given how many times Trump has told the public, including his supporters, things that are just not true, it is incumbent on him to demonstrate that there is nothing to this story.

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He could start by finally releasing his tax returns, and not just the two pages that MSNBC host Rachel Maddow shared with viewers in an over-hyped story. The questions remain: What are his interests? What are the connections between the Trump business empire and Russia? And how has this affected his positions on foreign policy?

Truly releasing his tax returns would go a long way toward solving some of the concerns about the sources of his money and taking a step toward transparency that has been absent until now. He should also step away from the ongoing Trump Twitter show, and level with Americans about his positions on Russia. If this is all about a serious effort to restart relations with Russia, he should let the public hear directly from him -- and in more than 140 characters.

Democrats will also have to prove their credibility to the public, meaning if there is a serious investigation and airing of information and it doesn't turn up any evidence of high crimes and misdemeanors, they too will need to show that they are willing to move on. If the Democrats turned this into a partisan witch hunt with no supporting evidence, then they would end up discrediting themselves.

Are we facing a scandal that's worse than Watergate? Republicans and Democrats should be hoping this is not the case. A scandal that grave is terrible for the entire republic, red and blue. But given all the doubts and misinformation that have characterized the last few months, the only way to find this out is for the Congress to conduct a full-throated investigation, for the FBI to be open and direct about what it's been doing, and for President Trump himself to finally prove his presidential muster and lift the cloud of suspicion through his own words and his own actions.