Four days later, she announced a formal impeachment inquiry.

"The times have found us," Pelosi stated and indeed they had. The tide was turning even as she spoke out against impeachment, as news that Donald Trump had pressured Ukraine to investigate Vice President Joe Biden, who is currently topping Democratic primary polls. In response to the news, seven Democratic representatives from vulnerable districts had come out in favour of impeachment and Pelosi soon followed.

In opening an official impeachment inquiry, Pelosi did something that has never been done in modern American politics: pursued impeachment not to punish past wrongs, but to prevent future ones - a sign of just how lawless the Trump presidency has become.

In the past 150 years, only two US presidents have faced impeachment hearings: Richard Nixon and Bill Clinton. Both men were in their second terms when their impeachment crises began. Members of Congress sifted through evidence of wrongdoing related to official acts - in the case of Nixon, efforts to discredit a government whistleblower and spy on political opponents, and in Clinton's, to cover up an affair.

In both cases, the focus was on the cover-up more than the crime. That's because the wrongdoing was in the past; the cover-up was the ongoing abuse of power.

In the case of Trump and Ukraine, the cover-up barely registers, because the crimes are ongoing. That became abundantly clear on Thursday, when Trump spoke to reporters on the South Lawn of the White House and encouraged both China and Ukraine to send him anti-Biden materials. Just as he had in 2016, when he encouraged Russia to find material on Hillary Clinton, Trump was openly encouraging foreign interference in a US election.

But unlike 2016, Trump has been using the powers of the presidency to make this happen. A shocking amount of evidence now points to this. When the impeachment inquiry was announced, the White House released both the rough transcript of Donald Trump's phone call with the Ukrainian president and the unclassified whistleblower report. Both showed Trump linked aid to Ukraine with an investigation into Biden. And now the US Special Representative to Ukraine has released a string of text messages that show a clear quid pro quo: the US would provide aid only if the Ukraine would investigate Trump's political rivals.

It is a breathtaking abuse of political power for personal gain and the American people seem to agree. Polling over the past few weeks shows steadily growing support for both impeachment and removal (a plurality or a majority, depending on which polls you look at).

The support for impeachment comes, at least in part, from how easy the story is to understand. It is a straightforward quid pro quo, do this for me and I'll do that for you. And it's one that is entirely consistent with Trump's previous actions.

Even Americans confused, or unpersuaded, by the Mueller report could remember Trump saying, "Russia, if you're listening ..." in his appeal to foreign interference in his first presidential campaign. The evidence is also clear and compelling: a few pages of phone transcripts, a crisply written whistleblower statement, a series of text messages.

That does not, however, mean that he will be removed from office. While the Republican support for Trump is eroding, it is doing so at a painfully slow pace and has not yet cost him a single vote in Congress. So long as the firewall of congressional Republicans and conservative media stands, he cannot be removed from office through ordinary processes.

And there is no reason to believe that his firewall will crumble. True, there are some fissures in his support. On Fox News, which has been an unfailing cheerleader for the Trump administration, the reporting teams have challenged conspiracy theories about the Ukraine whistleblower. Those challenges have led Trump to lash out - last weekend, he retweeted a series of attacks on Ed Henry, a reporter who pushed back on these conspiracies - but have not led Fox to more aggressively report on the scandal.

Yet even if Republican support remains unbroken, the impeachment inquiry is important. So long as the president invites foreign interference, the 2020 election cannot be considered a free and fair contest. Even the Federal Elections Committee has warned that accepting foreign help in an election is illegal - a warning the Trump administration has repeatedly ignored.

That is why the move toward impeachment matters, even though the odds are stacked against its success. American democracy has operated imperfectly over the past 250 years. It has often excluded entire classes of citizens and invited corruption. But a wholesale attack on the electoral process by a sitting president? That's something new and alarming. Democrats have finally started to challenge this existential threat to democracy. Whether they succeed depends on whether Republicans come along.