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Editor’s Note: Frank Bowman III is a law professor at the University of Missouri and author of “High Crimes and Misdemeanors; A History of Impeachment for the Age Of Trump.” The opinions expressed in this commentary are those of the author. Read more opinion on CNN.

(CNN) —  

The White House released a rough transcript Wednesday of a July phone call between President Trump and Ukrainian President Volodymyr Zelensky, indicating that the President pressured a foreign leader to gather dirt on a political opponent.

Frank Bowman III
U of Missouri School of Law
Frank Bowman III

As a result, we now have facts quite distinct from any that have come out about this President before – and the strongest, or at least most easily explainable, case for impeachment to date.

The allegations at the heart of special counsel Robert Mueller’s investigation into Russia’s interference in the 2016 election concerned Trump’s conduct while he was a candidate for office. There is good authority from the founding era that an effort to corrupt the electoral process ahead of an election might be impeachable. For example, at the 1787 Constitutional Convention, both George Mason and Gouverneur Morris observed that a president who “procured his appointment” by corrupting the electors must be impeachable.

But since impeachment is at its core about a president’s misuse of office or suitability to hold it, pre-inauguration conduct at least raises a tricky question. Whatever happened with the Russians during the 2016 election, Trump wasn’t then in a position to use the organs of the American state to encourage foreign interference.

Trump’s attempts to pressure Ukraine to investigate former Vice President Joe Biden, on the other hand, happened after Trump became President and had sworn an oath to faithfully execute his office and “preserve, protect, and defend the Constitution of the United States.”

With Russia, the most that can be said is that Trump expressed a willingness to receive political help from a hostile foreign power. Mueller could not prove there had been direct contact between the Trump campaign and Russian officials to coordinate that help. It remains troubling, if not impeachable, that the help was nonetheless delivered in the form of leaks and a social media misinformation campaign aimed at Trump’s opponent.

There are three key differences between the Russian and Ukrainian situations that should affect the impeachment debate.

One: This week, it has been revealed that Trump personally spoke with a foreign head of state and directly asked for a foreign government to probe for negative information about a possible presidential opponent. In other words, Trump’s call with President Zelensky may well constitute the very thing Trump denied throughout the Mueller investigation: “colluding” with a foreign power for personal electoral advantage.

Two: The fact that Ukraine is not a powerful traditional adversary, like Russia, makes the case worse in several ways. It means that Trump was not asking a geopolitical equal for help; he was demanding help from a weakened country situated on the border of an increasingly aggressive Russia; a country part of whose territory has already been illegally annexed by Russia, and whose continued survival as an independent nation depends on military, economic and diplomatic support from the United States and its European allies in NATO. How can the request of “a favor” from the American President to such a country be understood as anything but an extortionate demand?

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Three: Ever since the British invented impeachment in the 1300s, abuse of official power for personal gain has been on the short list of undeniably impeachable offenses in Great Britain and the United States. The second article of impeachment approved by the House Judiciary Committee against President Richard Nixon charged him with abuse of power. Nixon misused his domestic authority as President to get dirt on his political foes, and then used the powers of the federal government to try to cover it up.

But Nixon’s conduct was penny-ante compared to Trump’s. Trump didn’t cover up a second-rate burglary by a group of inept “plumbers” looking for dirt on Democrats. Rather, he appears to have wielded the entire economic, military and moral authority of a great nation to, effectively, extort another democratically elected head of state.

Leaving aside the question of impeachment, this episode must count as one of the most discreditable things any American President has ever done. Prior Presidents have been cruel or mean-spirited, bigoted or shortsighted, and sometimes exercised terrible judgment. And every President makes decisions with at least one eye on the political consequences. But I know of no comparable case where a President baldly, consciously misused the power of the whole nation for his own purely private political benefit, without even a credible claim that it was in the national interest.

If what Trump did here isn’t impeachable, nothing is.