The truth is, an impeachable offense is whatever the US Congress decides it is. That’s because impeachment isn’t really a legal response to perceived misconduct, it’s a political one.

And House speaker Nancy Pelosi has finally decided.

“The House of Representatives is moving forward with an official impeachment inquiry,” she said this afternoon. “The president must be held accountable. No one is above the law.”“

The catalyst was the news that Trump used military aid to Ukraine to leverage his efforts to uncover information that could be used against his potential Democratic rival for the presidency in 2020, former vice president Joe Biden.

Impeachment is a thing because the framers of the Constitution created an avenue to remove presidents, judges and other federal officeholders, even if the offense they are accused of isn’t addressed by the legal code. That’s why the impeachment article of the Constitution is so insanely vague: Impeachment, it says, is limited to “treason, bribery, or other high crimes and misdemeanors.”

To quote Benjamin Franklin: The articles of impeachment are needed to remove a president who has “rendered themselves obnoxious.” That’s hardly limited at all.

The only thing Congress needs in order to begin the impeachment process is the political will. And unfortunately for Trump, the political will is now there. At latest count 172 members of the House of Representatives have publicly said they would support an impeachment inquiry. A simple majority in the Democrat-led House would force the president to stand trial before the Republican-held Senate, where a two-thirds majority would be needed to convict.

Lawmakers don’t usually think impeachment is a good idea, even when a president has clearly broken the law. Former president Bill Clinton, for instance, was dishonest in grand jury testimony about his sexual relationship with a White House intern. Perjury is a crime, of course. But many lawmakers didn’t think the offense rose to the level of impeachment and Clinton was acquitted by the Senate.

Measured politically, impeachment ultimately becomes about proportional response. Lawmakers must decide if a presidential offense merits removal from office, and that the scale of it merits throwing the country into a political crisis. Usually, they don’t think so. Trump’s efforts to obstruct the investigation into his campaign’s ties with Russia was, for instance, while unseemly, not enough for the House to move toward impeachment.

Trump’s most recent scandal, however, is changing some minds on Capitol Hill. And while it looks bad, it might just be the sum total of the offenses Trump has racked up that is changing the tide. The details of the latest scandal remain secret. And a lot of what the public does know is based on media reports quoting anonymous sources. What’s been confirmed by the Trump team’s own admission, however, could be enough.

The reason for impeachment

Here’s what is for certain: Trump withheld military aid to Ukraine at some point over the summer, blindsiding Ukrainian officials. A short while later, on July 25, he phoned up Ukrainian president Volodymyr Zelensky. The president pressured his counterpart to investigate Biden’s calls for Ukraine to dismiss one of the country’s notoriously corrupt prosecutors. Trump says he believes Biden—widely viewed as his most likely 2020 election opponent—was improperly trying to protect his son Hunter, who had business interests in Ukraine This theory, it’s worth noting, has been soundly disproved. So it looks a lot like the president was searching for dirt on his potential rival.

Withholding military aid from a foreign ally to leverage a domestic presidential campaign is unscrupulous at best, and damaging to US democracy at worst. But illegal? Hard to say.

This all came to light when someone in the intelligence community—now known only as “the whistleblower”—flagged Trump’s phone call, wrote up an official complaint, and sent it to his boss. The boss deemed it an “urgent concern” and passed it along to his own boss: Trump’s director of national intelligence, Joseph Maguire. Maguire should have notified Congress within a week. He didn’t, which appears to be pretty shady. Now it doesn’t matter because the whistleblower wants to speak with lawmakers directly. That will likely happen later this week.

The impeachment process

Launching an impeachment inquiry means either the House Judiciary Committee or an appointed special committee will begin to investigate Trump’s communications with Ukraine, and build evidence to support an impeachment. Eventually it will come to a vote in the full House.

If a majority of the House votes on an article of impeachment, which is still a long shot, the president is impeached. The case then moves to the Senate, where these extraordinary trials pull in every branch of government.

The chief justice of the Supreme Court will act as judge (right now that’s John Roberts). Members of the House will act as the prosecution. The president’s own lawyers will mount a defense. The Senate acts as the jury. If two-thirds decide the president is guilty, he would be removed from office.

And then Mike Pence becomes president.