House Speaker Nancy Pelosi, who is the most senior Democrat, announced official proceedings via a press conference. "The president must be held accountable. No one is above the law," she said.
Pelosi had previously voiced that the Democrats would not take action against President Trump until they had the best possible case to support an impeachment inquiry.
Trump's latest scandal involving a call to the Ukraine has given them the evidence they were looking for.
A whistleblower reportedly filed a complaint over an exchange Trump had with Ukrainian president Volodymyr Zelensky back on July 25, whereby the President supposedly pressured Zelensky eight times to dig up damaging information about his political rival Joe Biden and his son Hunter Biden.
At the time, the Ukraine was expecting a $US391 million military aid package from the United States, which they later eventually in August.
"This week the president has admitted to asking the president of Ukraine to take action that would benefit him politically," said Pelosi. "The president must be held accountable."
Not worried that there was anything inappropriate about his behaviour and conversation, Trump has agreed to release transcripts of the call.
But he hasn’t done so quietly.
Lashing out on Twitter, he said, "You will see it was a very friendly and totally appropriate call. No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!"
It's a long road to impeachment, even if the evidence supports claims that Trump breached his constitutional responsibilities. For a president to be removed from office, they must be impeached by a majority vote of the US House of Representatives, then convicted by a two-thirds majority of the US Senate.
No president has ever been removed from office as a result of this process.