WATCH: Harvard Law professor Laurence Tribe explains why the Constitution requires ability to indict sitting president

Constitutional law expert Laurence Tribe -- who has taught at Harvard Law for fifty-years -- explained why constitutional imperatives supersede Department of Justice policy on the question of whether a sitting president can be indicted while in office.

Tribe joined MSNBC anchor Lawrence O'Donnell to discuss his Monday op-ed in the Boston Globetitled, "Constitution rules out a sitting president’s immunity from criminal prosecution."

"The president and vice president run as a ticket. No president selects a vice president who wouldn’t strongly consider doing for him exactly what Vice President Gerald Ford did for President Richard Nixon: namely, give the president a full pardon shortly after he becomes the former president — whether that sudden reversal of fortune occurs upon the president’s being turned out by the voters, or upon his being impeached and removed, or upon his resigning under the threat of such ignominious removal," Tribe wrote.

"It’s crazy to assume that the framers of the impeachment power would have created a system in which even the most criminally corrupt president could permanently escape full accountability," he added.

O'Donnell explained reality to Trump in the opening of the segment.

"So what would happen if Donald Trump went out on Fifth Avenue and shot someone?" O'Donnell asked.

"During the presidential campaign he said he could do that and his supporters would still vote for him and that might be true of his supporters, but what's also true is that the NYPD would immediately either shoot and kill Donald Trump on the spot or arrest him and charge him with murder," O'Donnell predicted.

"The ordinary sequence might well be to impeach and remove first, but if a president has committed a crime in order to get to office and has combined the office and his business interests in a way that might be inimical to the United States in order to enrich himself and his family, then the only way to carry out the Constitution's design without making the pardon power and the guaranteed liability to criminal punishment clash with one another is to remain open to the possibility of indicting and trying the president while he is in office," Tribe explained.

"And it seems to me that that's the only coherent way to read the Constitution and to understand its history, because the framers said they were concerned about the corrupt acquisition of presidential power," he continued. "The incentives would be all the wrong way if a president could bribe electors or hide things from the voters in order to become president and then be guaranteed that he would never serve a day in jail."

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