“The president must be held accountable,” she said. “No one is above the law.”

Mr. Trump is fighting back.

Impeachment 101: If the House votes on a simple majority to impeach, the proceedings then move to the Senate. If at least two-thirds of the senators find the president guilty, he is removed and the vice president takes over.

Only three presidents have been subjected to impeachment proceedings. (To date, impeachment has never resulted in the removal of an American president from office.)

A tipping point: Democrats who had avoided invoking impeachment proceedings turned after learning of a whistle-blower’s complaint — which relates to Mr. Trump’s efforts to get the Ukrainian government to investigate former Vice President Joseph R. Biden Jr. and his son, Hunter Biden.

As part of a broader effort to quell calls for Mr. Trump’s impeachment, White House and intelligence officials are now working out a deal to allow the whistle-blower to testify, following the whistle-blower’s attempts to meet with members of Congress. Part of this effort includes releasing the transcript of a call between Mr. Trump and Ukraine’s president.