Despite House Speaker Nancy Pelosi’s (D-Calif.) best efforts, House Democrats are seriously considering beginning impeachment proceedings against President Trump.

The case for impeachment has been building over the past month. Here’s a run-down of some key flash points that have brought an increasing number of Democrats (and one Republican) on board:

The Mueller report is released: There were some Democrats who thought that the obstruction-y actions outlined by special counsel Robert S. Mueller III in his report released April 18 were enough to impeach the president. But the majority stuck with Pelosi, who privately urged her caucus to take it slow.

Elizabeth Warren takes the stage: She was one of the first 2020 Democrats to come out in favor of impeachment and gave an eloquent, forceful explanation why at a CNN town hall April 22 that drew headlines and became a bumper-sticker argument for the pro-impeachers: “There is no ‘political inconvenience’ exception to the United States Constitution.”

The Mueller letter comes out: On April 30, a letter is leaked that showed that Mueller was unhappy with how Attorney General William P. Barr characterized the special counsel’s report. Mueller sent that letter to Barr in March, urging him to release more and warning of “misunderstandings.” This followed reports that Mueller’s team thought Barr’s summary had been too easy on Trump, and it gives the perception among some Democrats that there is a coverup in the Trump administration. Speaking of . . .

Trump blocks a lot of congressional investigations: Impeachment talks were continuing in Democratic circles, but the lawmakers who mattered — Pelosi and Democratic leaders of key committees — had settled on another plan. They were investigating all aspects of Trump’s life, including the Mueller report, without starting impeachment proceedings. “Impeach or nothing. No, it’s not that,” Pelosi would later explain.

But Trump had other ideas. “We’re fighting all the subpoenas,” he said. “These aren’t, like, impartial people. The Democrats are trying to win 2020.”

He sued his accounting firm and Congress to prevent them from getting his financial information. His treasury secretary said he won’t release Trump’s tax returns, despite an IRS interpretation of the law otherwise. His former top aides refused to comply with subpoenas to testify. If Trump was stonewalling traditional investigations, what other options did Congress have but to consider impeachment, some Democrats wondered.

“I think what he’s trying to do and I think what they think he’s trying to do is render us toothless and say we’re not that important,” said House Financial Services Committee Chairwoman Maxine Waters (D-Calif.), who was one of the first lawmakers to call for impeachment.

Attorney General Barr doesn’t show up, so Congress holds him in contempt: A sitting Cabinet secretary refused to come to Congress to testify about the Mueller report. Barr said his problem was with the format, but it added to the perception that the White House was blocking Congress from its constitutionally mandated role of oversight.

A statue of a chicken sits above the nameplate and near the empty seat of U.S. Attorney General William Barr, who was scheduled to appear at a House Judiciary Committee hearing May 2 but didn't show. (Clodagh Kilcoyne/Reuters)

On May 8, the Democratic-led House Judiciary Committee voted to hold Barr in contempt of Congress for not showing up. He is only the second sitting attorney general in U.S. history to be held in contempt. But if this was a big deal symbolically, practically speaking, it didn’t change much. Barr still hasn’t testified in front of Congress.

Pelosi says Trump is “becoming self-impeachable”: The morning of the contempt vote, Pelosi sat at a Washington Post Live event and said Trump is “becoming self-impeachable.” It wasn’t immediately clear what she meant — that he was making his own case that he should be impeached, or that he was damaging his election chances by blockading Congress. But it did seem like a turning point in Pelosi’s language, that she was more open than ever to considering impeaching Trump. At least, we wrote, she’s laying the groundwork to consider impeaching Trump if it comes to that.

Justin Amash is open to impeachment proceedings against Trump: A sitting Republican member of Congress, Rep. Justin Amash (R-Mich.), tweeted over the weekend that he had finished reading the Mueller report and he thought Trump “engaged in impeachable conduct.” This one Republican wasn’t going to change Pelosi’s calculations to avoid impeachment, but suddenly, Democrats’ impeachment efforts were technically bipartisan.

Donald McGahn ignores a subpoena to talk to Congress: On Tuesday, another big name in Trump World refused to show up to Congress. McGahn was the White House’s top lawyer, and he is a key witness in the Mueller report to Trump’s attempts to fire the special counsel then lie about it. The House Judiciary Committee issued a subpoena to force him to talk, but McGahn, under pressure from Trump, did not show up.

"We will hold this president accountable,” House Judiciary Committee Chairman Jerrold Nadler (D-N.Y.) said at the hearing, with an empty seat where McGahn was supposed to be, “one way or another.”

A seat at the witness table sits empty prior to a House Judiciary Committee hearing in which former White House Counsel Don McGahn was subpoenaed to testify. (Alex Wong/Getty Images)

To many Democrats on the fence about impeachment, this was the last straw. The night before, Nadler and top Democrats had urged Pelosi to consider impeachment proceedings, even if it doesn’t lead to articles of impeachment against the president.

What other choice does Congress have at this point, they argued? If potential crimes outlined in the Mueller report weren’t deserving enough, the cover up has been, they argued.

Pelosi accuses Trump of a coverup and says that “could be an impeachable offense”: She made the remarks at a gathering of progressives on Wednesday, just hours after tamping down impeachment calls from her party in a private meeting. It was the most open to impeachment that Pelosi has ever publicly been.