"The president must be held accountable," Pelosi said.
"No one is above the law." Pelosi has faced mounting pressure from her membership, which swung strongly in favor of pursuing impeachment as new details have come to light.
Trump is accused of urging Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden and his son, while withholding nearly $400 million in military aid to the country.
Trump has called Democratic inquiries into the matter a "witch hunt," but acknowledged discussing Biden and "corruption" in a July 25 phone call with Zelensky.
"We had a great conversation," the president said on Sunday.
"It was largely the fact that we don't want our people, like Vice President Biden and his son, creating to [sic] the corruption already in the Ukraine." Biden issued a statement earlier on Tuesday, saying if Trump continues to stonewall a House investigation, then Congress would have "no choice but to initiate impeachment." "I can take the political attacks.
They'll come and go and soon be forgotten," Biden said.
"But if we allow a president to get away with shredding the United States Constitution, that will last forever." Rep.
Adam Schiff, the chair of the House Intelligence Committee, issued a subpoena on Sept.
13 for a whistleblower complaint, believed to relate to the Ukraine phone call.
The Trump administration has refused to turn over the complaint, but Trump on Tuesday said he would release a "fully declassified and unredacted transcript" of the call on Wednesday.
Pelosi has been trying to hold the line against impeachment for months, as her caucus remained deeply ambivalent about it.
But the Ukraine scandal has prompted at least 34 members -- including several freshmen in vulnerable districts -- to join calls for impeachment.
The cable networks kept on-screen tallies of Democrats who have called for impeachment on Tuesday.
CNN counted 163 by the time of Pelosi's announcement, while MSNBC pegged the figure at 179.
John Lewis came out in support earlier on Tuesday.
"Now is the time to act," Lewis said on the House floor.
"I have been patient when we tried every other path and used every other tool.
I truly believe the time to begin impeachment proceedings against this president has come." Schiff said his committee has been in touch with the whistleblower's attorneys, who have said their client wants to talk to the committee.
Schiff is seeking to arrange the whistleblower's testimony as soon as this week.
Joseph Maguire, the acting Director of National Intelligence, is scheduled to appear before the committee on Thursday.