Paul Begala, a Democratic strategist and CNN political commentator, was a political consultant for Bill Clinton's presidential campaign in 1992 and served as a counselor to Clinton in the White House. The opinions expressed in this commentary are his. View more opinion articles on CNN.

(CNN) "When the President does it, that means it is not illegal." So said Richard Nixon in his famous post-presidency interview with David Frost.

Nixon was no JFK. His words did not soar; they aren't etched in marble the way Kennedy's are. His prose was more prosaic. But when Nixon tried to justify criminal conduct by claiming a president is above the law, he earned his place in the compendium of deathless presidential quotations.

Of course, we remember Nixon's words for the same reason we remember Louis XIV saying, "L'etat c'est moi" -- because both are an outrageous assertion of absolute power. Free people for centuries have rebelled at the notion that the ruler is above the law. In the Anglo-American tradition, the concept that even a king must be accountable to the law goes back to the Magna Carta in 1215.

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In his defense of President Donald Trump, attorney and Harvard Law School professor emeritus Alan Dershowitz added a third variant to Nixon and Louis XIV : If a President thinks his re-election is in the public interest, anything he does in pursuit of his re-election is legal.

Seriously. As part of the Senate impeachment trial, Dershowitz was called upon to answer a question about the legality of a quid pro quo -- that well-worn Latin phrase that means, literally, "this for that." House Democrats have impeached President Trump for allegedly seeking to exchange $391 million in Ukrainian military aid (this) for Ukraine announcing it would investigate supposed corruption by his political rival, former Vice President Joe Biden and his son (that).

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