"Today, I'm announcing the House of Representatives is moving forward with an official impeachment inquiry. I'm directing our 6 committees to proceed with their investigations under that umbrella ... The president must be held accountable."

Why it matters: This is a watershed moment. 31 months after the first House Democrat called for his ouster, President Trump faces the formal impeachment inquiry that could define the rest of his presidency.

Pelosi has resisted calls to impeach Trump for more than a year, even as the number of House Democrats demanding a formal inquiry crossed into the majority in June.

The big picture: The allegations that Trump may have pressured the president of Ukraine to investigate Joe Biden opened the floodgates this week for swing district Democrats and influential holdouts to endorse impeachment, finally pushing Pelosi over the edge.

More than 20 House Democrats endorsed impeachment on Tuesday alone, including civil rights icon Rep. John Lewis (D-Ga.). Democrats remain ~50 votes shy of the 218 to impeach, but momentum is moving in that direction.

Behind the scenes: Pelosi now sees a potential necessity for impeachment even if she dislikes the political impact it could have.

One factor driving Pelosi's thinking: The White House has refused to turn over the whistleblower's complaint about Trump's actions.

The White House has refused to turn over the whistleblower's complaint about Trump's actions. Another factor: Pelosi is working to protect vulnerable Democrats who aren't on board with impeachment — assuming they get past the necessary 218 votes needed to initiate formal proceedings.

Between the lines: The White House still has a potential way to halt the escalating momentum for impeachment: turn over the whistleblower report.

The White House is releasing the transcript of Trump's call with Ukraine's new president, but that won't go far enough.

the transcript of Trump's call with Ukraine's new president, but that won't go far enough. The chairmen of 3 key Democratic committees investigating Trump have demanded the White House turn over documents related to Biden and Ukraine by Thursday, threatening "escalated measures" if the administration refuses to comply.

investigating Trump have demanded the White House turn over documents related to Biden and Ukraine by Thursday, threatening "escalated measures" if the administration refuses to comply. And the GOP-controlled Senate voted unanimously to urge the administration to release the whistleblower complaint to the Senate Intelligence Committee.

What to watch: House Democrats tell Axios that there will not be a House vote on the formal impeachment inquiry. But they add that this gives ongoing investigations more urgency and emphasis and will speed up the process.

Go deeper: Inside Pelosi's thinking on impeachment

This is a developing news story. Please check back for updates.