Lawmakers in the Democrat-led U.S. House of Representatives have begun a formal debate on impeaching President Donald Trump that is expected to last about six hours and is likely to end in the approval of two articles of impeachment.
If the lower house backs impeachment over the opposition of Trump's fellow Republicans in the minority, it would mark just the third time in U.S. history that a president has been impeached.
Many of the president's defenders in the Senate have already vowed to back him if the impeachment goes to trial in the Republican-controlled upper house, which would be the next step under the U.S. Constitution.
The momentous session quickly got bogged down in procedural questions early on December 18, but cleared a major hurdle when rules were established for six hours of debate.
"I solemnly and sadly open the debate on impeachment of the president of the United States," Democratic House Speaker Nancy Pelosi (Democrat-California) told her fellow lawmakers in the 435-member House. "If we do not act now, we would be derelict in our duties."
She said Trump represented an "ongoing threat" to U.S. national security.
The lower chamber is expected to approve two impeachment charges -- abuse of power and obstruction of Congress -- related to Trump's dealings with Ukraine.
Trump is accused of pressuring Ukrainian President Volodymyr Zelenskiy to investigate a political opponent, former Vice President and current presidential hopeful Joe Biden, by withholding congressionally approved military aid.
Critics say the "favor" that he urged from Zelenskiy amounted to an invitation from a U.S. president for foreign interference in a U.S. election.
Trump has already announced his intention to run for reelection in 2020.
Just two of history's 44 previous U.S. presidents have ever been impeached -- Andrew Johnson in 1868 and Bill Clinton in 1998 -- and neither was convicted or removed by the Senate.
Ahead of the start of the debate in the House on December 18, Trump reiterated on Twitter he had done "nothing wrong."
While historically low, Trump's public-approval ratings have been remarkably stable since his surprise victory in 2016 as a billionaire political outsider pledging to curb immigration, reshape or walk away from international agreements on trade, the environment, and nuclear activities, and "drain the swamp" in Washington.
Statistical analyst FiveThirtyEight's tracker of impeachment polls last week showed 48 percent of Americans in favor of impeachment and 44 percent opposed.
In a six-page letter released on December 17, Trump accused Pelosi of pursuing an attempted "coup" that is "subverting America's democracy."
The letter was released as the House Rules Committee approved legislation that provides for six hours of debate, followed by final votes on the charges against Trump.
Pelosi wrote to colleagues that impeachment was "one of the most solemn powers granted to us by the constitution," and called it a "very prayerful moment in our nation's history."
Senior Republican Senator Lindsey Graham (South Carolina) told reporters on December 18 that he would aim for a short trial in any impeachment trial.
"When it gets here, my goal is to have as short a trial as possible," Reuters quoted Graham as saying to reporters. "I am not going to support witnesses being called for by the president. I am not going to support witnesses being called for by [Democratic leader] Senator [Chuck] Schumer."
Senate Majority Leader Mitch McConnell (Republican-Kentucky) said recently he would "take my cues from the president's lawyers" and that "there is no chance the president is going to be removed from office."
The U.S. Constitution grants Congress the authority to impeach and remove presidents, vice presidents, and other civil officers for "treason, bribery, or other high crimes and misdemeanors," a term which is not specifically defined.