After resisting demands from many party members, Pelosi finally agreed on Tuesday to start the process, even if it ultimately will be symbolic as the Republican-controlled Senate is unlikely to vote Trump out of office, while deepening the political divide.
The impeachment is a continuation of the allegations of collusion with foreigners in election interference made against Trump.
The new allegations that the House will investigate in the impeachment process relate to Trump asking the Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden and his son over their business dealings and had frozen $391 million in aid to that country a few days before his conversation with Zelensky.
Since Biden is a candidate for Democratic Party nomination to run against Trump next year, the Democrats consider it an attempt to get a foreign country to help his campaign.
Pelosi said: "The actions taken to date by the President have seriously violated the Constitution."
One of the reasons cited for going ahead with the impeachment was that Trump had refused to provide Congress investigating the matter a transcript of the conversation he had with Zelensky. But Trump released the transcript on Wednesday and it did show that he had made the request for a probe.
According to a reconstruction of the conversation in July that was presented as a transcript, Trump told Zelensky: "There is a lot of talk about Biden's son, that Biden stopped the prosecution, and a lot of people want to find out about that. So whatever you can do with the attorney general would be great."
It did not show that Trump had discussed the aid freeze.
He had also asked for "a favour" to give him information about the hacking of the Democratic Party's computer server.
Pelosi said: "The release of the notes of the call by the White House confirms that the President engaged in behaviour that undermines the integrity of our elections, the dignity of the office he holds and our national security."
Trump has responded by calling it "Presidential Harassment" and his party has begun using the impeachment threat as a fund-raising ploy and trying to strengthen the Republican base.
The allegations about the phone call were made by an unidentified government employee who listened to the conversation or had seen documents about it. The employee had filed a whistle-blower complaint against Trump, which the administration did not forward to the Congress.
Democrats had earlier alleged that the Trump campaign had collaborated with the Russ, who interfered in the 2016 elections.
An inquiry by a special counsel failed to establish such a collaboration, but left open possibility that he had tried to obstruct the probe.
Many Democrats clamoured for impeaching Trump on the obstruction charge, which Pelosi resisted, but now the Ukraine allegations give a stronger case.
Only two Oresidents, Andrew Jackson in 1868 and Bill Clinton in 1998, have been impeached. But Clinton was not convicted by the Senate and completed his term.
Despite its formidable sound, impeachment merely means framing charges, which will be put in a document called Articles of Impeachment and sent to the Senate for formal prosecution and decision on conviction.
Biden may end up a collateral casualty of the impeachment as it gives a view of the murky world of politics and diplomacy and brings to the fore allegations against his family.
Biden's son, Hunter, is a former cocaine addict with arrest record who was removed as a Naval Reserve office because of testing positive for drug use.
But after going into business with deals in China and Ukraine, he became wealthy.
An Ukranian prosecutor began investigating the gas company Burisma on whose board Hunter Biden served. But the prosecutor was removed and later Biden is reported to have boasted he was responsible for it, though it was only because it was a barrier to developing the nation's economy.
He had allegedly threatened to withhold aid if the probe went ahead; a similar charge about aid is now being levelled against Trump.
Trump's personal lawyer, Rudy Giuliani, began an investigation into the affair.
Trump has said that Hunter Biden's involvement in Ukraine and Chinese business deals would compromise his father's ability to protect US interests.
According to Politico, Biden's poll numbers have been slipping in the states which will be holding early primaries or caucuses for the Democratic party's presidential nomination. In one of them, New Hampshire, Biden's support fell by 18 points, it said.
The impeachment could tie up the Democrats during the politically crucial year, even though they could tie it to the broader campaign against Trump in the 2020 election.
It could, on the other hand, strengthen the Republicans by rallying even the waverers to Trump's side.
(Arul Louis can be contacted at arul.l@.in and followed on Twitter @arulouis)
( With inputs from IANS )