Special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE is tentatively set to testify about his 448-page report before House Judiciary Committee on May 15.

The report, of course, details his investigation into Russian interference in the 2016 presidential election and whether President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE violated criminal obstruction of justice laws.

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So, what should the public watch for in Mueller’s testimony? Here are four things that Congress needs to ask Mueller:

1. Mueller should set the record straight for the American public on Russian interference in our elections. Mueller, of course, will stand with our national security experts — not Russian President Vladimir Putin Vladimir Vladimirovich PutinNavalny released from hospital after suspected poisoning Ex-Trump national security adviser says US leaders 'making it easy for Putin' to meddle The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting MORE — on how Russia attacked our electoral process.

This will stand in stark contrast to Trump, who recently spent an hour on the phone with Putin kvetching about what he calls the “Russian hoax.”

“Hoax” could not be further from the truth. What Russia has done and will do again amounts to an attack on the United States, which our commander-in-chief should fight against, not outright deny. Congress should ask Mueller for his views on the nature and severity of the threat going forward, and what is — and is not — being done about it. Mueller was FBI director for over a decade, after all, and knows his stuff. He might not opine on the ongoing threat publicly, but the question is a vital one for the integrity of our country’s electoral process.

2. Mueller needs to clear up why he declined to make a call on whether the evidence of obstruction by the president rose to the level of proof beyond a reasonable doubt.

Barr testified that Mueller reassured him that Mueller’s equivocation had nothing to do with the DOJ policy against indicting a sitting president. The Mueller report suggests the very opposite — that he declined to give a thumbs-up to obstruction charges because the DOJ policy made it impossible for the president to defend himself against such charges in court.

If Trump can’t be indicted, he can’t wage a defense on the facts. This disparity between Barr and Mueller needs to be cleared up. Mueller might also be asked to clarify why he wrote that the report did not “exonerate” Trump, because there is a lot of potential evidence of obstruction in there.

3. It would help the public if Mueller underscored the statement in his report that he did not reach a decision on “collusion” — because collusion is not even a legal term.

Barr has collapsed the term “collusion” with the legal concept of conspiracy. This legal slip-up amounted to attorney general malpractice. Legal terms denoting crimes have precise meanings for a reason: The government bears the burden of proving the elements making up a crime beyond a reasonable doubt in court.

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Collusion is just a word. People can collude without conspiring. There is nothing in the report that “exonerates” Trump of the colloquial term, collusion. Congress has the constitutional prerogative to further investigate that issue, although it might come to no consequence other than informing the public that the threat of Russian attacks on the electoral process remains real.

4. Congress should ask Mueller for his views on whether the special counsel regulations need amending, and whether Congress should pass legislation protecting the process — as it did post-Watergate. For those who critique the very propriety of the House of Representatives’ inquiries, the Constitution itself has the answer. A line of questioning regarding DOJ’s process for investigating presidents is central to Congress’s own authority to investigate. The U.S. Supreme Court has linked this to Congress’s express constitutional power to legislate under Article I of the Constitution.

In today’s black-and-white world, Mueller can highlight the sobering shades of gray that continue to shade Trump, Russia and the question of obstruction of justice. Despite Barr’s unfortunate talk of Mueller team snittiness, Mueller himself remains an impeccably credentialed and independent “rule-of-law” guy. He is an old school Republican who cares about the institution of the administration of justice in the United States. Hopefully, his testimony can remind us all that facts and law — not ideology and power — can still carry the day.

Kim Wehle is a former assistant U.S. attorney and a former associate independent counsel in the Whitewater investigation. Wehle is a professor at the University of Baltimore School of Law. Her book, “How to Read the Constitution and—Why,” will be published in June. Follow her on Twitter @kim_wehle.