Six months ago, Congressional Democrats hemmed and hawed about what to do with Robert Mueller’s more than 400-page report on Russian intervention in the 2016 election, which was filled with numerous instances of possible obstruction by President Donald Trump. (While Mueller did not determine whether Trump had committed obstruction, Attorney General Bill Barr concluded he had not.)

But it was a 30-minute-long phone call between Trump and the Ukrainian president in July — which may not have broken any specific laws — that proved the tipping point for Democrats on opening an impeachment inquiry.

The stunningly fast swell of support for an impeachment probe after a whistleblower submitted a complaint reportedly based on the call has revealed an important development in how members of Congress, particularly House Democrats, are beginning to define a key constitutional power afforded to them: deciding what constitutes “high crimes and misdemeanors,” the grounds for impeachment enshrined in the Constitution. Moving forward with an inquiry on the basis of Trump’s call with Ukrainian President Volodymyr Zelensky shows that they are operating under the principle that an impeachable offense need not be an actual crime.

“No one should have any illusions about the seriousness of what is already uncontested, and that is the President of the United States has betrayed his oath of office and sacrificed our national security in doing so,” House Intelligence Committee Chairman Adam Schiff, a Democrat from California, said on Wednesday. “And that I think is quintessentially what the Framers were concerned about. I think it’s quintessentially what the Framers thought was the sum and substance of what might warrant a president’s removal from office.”

Legal experts say Trump’s call with Zelensky, in which he asked the Ukrainian president to investigate Joe Biden’s son Hunter and may have implicitly tied foreign aid money to the request, may not have violated the letter of the law. “That would be politicizing law enforcement investigations and particularly politicizing a foreign law enforcement investigation for personal political gain,” says national security lawyer Bradley Moss. “I don’t know if the law has quite caught up to that idea, because we haven’t really contemplated the notion.”

In a declassified memorandum of the July 25 call released by the White House on Wednesday, Trump noted, “I wouldn’t say that it’s reciprocal necessarily because things are happening that are not good but the United States has been very very good to Ukraine,” according to the memo, which is not a direct transcript of the conversation. Then Trump asked Zelensky, “There’s a lot of talk about Biden’s son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great. Biden went around bragging that he stopped the prosecution so if you can look into it… It sounds horrible to me.” Trump had ordered almost $400 million in military aid to be withheld from Ukraine at least a week before this phone call, the Washington Post reported.

As the Democrats moved toward impeachment in the days preceding the release of the memo about the call, legal experts told TIME that it would probably be tricky to point to any specific law broken in the conversation, especially given the President’s broad powers to conduct foreign policy under Article II of the Constitution. Multiple experts in the Foreign Corrupt Practices Act, which prohibits bribing foreign officials to further business deals, said this situation likely wouldn’t apply, because it doesn’t involve payment to an individual government official and isn’t for business purposes. Questions over whether federal bribery statutes were violated would turn on proof that Trump was seeking a quid pro quo, and whether an investigation into Biden would be considered “anything of value” under the law.

“There is no quid pro quo necessary to betray your country or your oath of office,” Schiff said Wednesday.

It was revealed the same day that the call memo was released that the Justice Department had been focused on whether the call represented a campaign finance violation, but determined it had not. Department Spokesperson Kerri Kupec said in a statement on Wednesday that the Department of Justice had been made aware in August that a conversation between Trump and Zelensky marked “a potential violation of federal campaign finance law,” according to a letter from the Inspector General for the Intelligence Community to the Director of National Intelligence about a whistleblower complaint, which was then referred to the Justice Department. “Relying on established procedures set forth in the Justice Manual, the Department’s Criminal Division reviewed the official record of the call and determined, based on the facts and applicable law, that there was no campaign finance violation and that no further action was warranted,” Kupec said. ��All relevant components of the Department agreed with this legal conclusion, and the Department has concluded the matter.”

“It turned out to be a nothing call,” Trump said on Wednesday.

It has not been definitively determined if Trump violated any laws in this conversation or in other situations that resulted in the whistleblower complaint. But even if some conclude that he has broken the law, the standing view of the executive branch is that a sitting President can’t be indicted.

All of which is why Asha Rangappa, a former FBI special agent who is now a senior lecturer at the Jackson Institute for Global Affairs, argues that asking if a President broke the law or not isn’t necessarily the right question when assessing if something is an impeachable offense. “Either you have laws that don’t cover the specific kind of behavior that he is engaged in, because it only becomes problematic if it’s the President engaging in it, or you’re dealing with things that are just much bigger in the sense of abuse of power, or violations of oath of office or bigger constitutional principles that are being violated, even if they don’t necessarily violate a specific aspect of the U.S. code,” she says. “We can start drilling down into well, is this bribery or Foreign Corrupt Practices Act? But I feel like those miss the point.”

In other words, Rangappa believes setting criminality as the test for impeachment may lead people to miss the forest for the trees in terms of overall presidential conduct and potential abuse of power. “The collusion conversation kind of got hijacked, and partly because we accepted that criminality was the bar,” Rangappa says. “So as long as someone doesn’t cross that bar, as long as Trump doesn’t cross the bar of criminality, somehow that’s okay. And I think that we learned our lesson once: that’s not the bar.”

Impeachment of a President is an extraordinary step that has only been attempted three times in U.S. history, and no president has ever been removed from office by the Senate through the process. (This includes President Richard Nixon, who resigned before the House’s impeachment vote.) The debate over what constitutes an impeachable offense is robust. But the idea that impeachable conduct need not be an actual crime is well established. In the Federalist Papers, Alexander Hamilton wrote about impeachment in Federalist No. 65, “The subjects of its jurisdiction are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself.”

In the modern era, Congress has continued to interpret the Constitution in this way. One of the articles of impeachment drafted for Nixon was for “abuse of power.” And a Congressional Research Service report on impeachment and removal prepared in October 2015 says, “Impeachable conduct does not appear to be limited to criminal behavior. Congress has identified three general types of conduct that constitute grounds for impeachment, although these categories should not be understood as exhaustive: (1) improperly exceeding or abusing the powers of the office; (2) behavior incompatible with the function and purpose of the office; and (3) misusing the office for an improper purpose or for personal gain.”

In the end, impeachment isn’t a legal process. It’s a constitutional one, but it’s a fundamentally political one, too. Which perhaps explains best why Trump’s call with Zelensky proved more compelling to House Democrats than any other conduct revealed in the Mueller report or over the course of his presidency. “We have many other, shall we say, candidates for impeachable offense in terms of the Constitution,” House Speaker Nancy Pelosi said at the Atlantic Festival on Tuesday just hours before she announced the formal impeachment inquiry. “But this one is the most understandable by the public.”

Write to Tessa Berenson at tessa.berenson@time.com.