House Speaker Nancy Pelosi (D-California) announced Tuesday afternoon that Congress’s lower chamber will start a formal impeachment investigation into President Donald Trump.

The development comes as Democrats have been pushing to impeach Trump in response to allegations that he pressured Ukraine’s government to probe Joe Biden and his son for alleged corruption during a phone call with the country’s president in July. There are now 172 lawmakers — including more than two-thirds of House Democrats and one Independent — who have publicly said they support impeachment proceedings against Trump. However, Democrats are likely to face opposition from the Senate, which is under Republican control.

On Tuesday, Pelosi — who for months has voiced hesitation to support impeachment — met in private with Democratic caucus members and with several leaders of six committees who have launched inquires into Trump over several affairs. The House speaker is also mulling the formation of a special committee to investigate Trump’s negotiations with Ukraine and to possibly establish preparation for impeachment articles based on the conclusions. The panel would be similar to the one the Senate formed in 1973 to probe former President Richard Nixon’s involvement in the Watergate scandal.

An impeachment inquiry would mark a watershed moment in American history, as many lawmakers have said a sitting U.S. president using help from a foreign government to investigate a political opponent is practically tantamount to treason. Biden, the Democratic front-runner in the 2020 presidential election, also said Tuesday he is in favor of beginning impeachment proceedings against Trump.

Another key piece of information that could likely help bolster House Democrats’ impeachment case is the transcript of Trump’s phone conversation with Ukrainian President Volodymyr Zelensky. Trump said on Tuesday that he would authorize the release of the transcript and tweeted that the call with Ukraine’s leader was “very friendly and totally appropriate.”

It’s also important to note that the start of a formal impeachment investigation does not necessarily entail that House Democrats will vote to charge Trump with any crimes or misdemeanors.