House debate begins at 9am (2pm UK) Trump says impeachment probe had "less due process than the Salem Witch Trials" Polls show increase in support for Trump's impeachment The US impeachment process explained Donald Trump is set to become just the third president in US history to be impeached, with a historic debate due to begin in the House of Representatives on Wednesday. The 435-member chamber will debate two articles of impeachment against Mr Trump, which charge him with abusing power and obstructing Congress. Since the Democrats hold the majority in the House, Mr Trump is all but certain to be impeached. Ahead of the historic vote, the president tweeted: "Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible Thing. Read the Transcripts. This should never happen to another President again. Say a PRAYER!" In a furious letter to the Democrat House Speaker Nancy Pelosi, the president denounced the "vicious crusade" against him, but acknowledged he was powerless to stop the expected outcome. "When people look back at this affair, I want them to understand it, and learn for it, so that it can never happen to another president again," he wrote. Ms Pelosi, who warned earlier this year against pursuing a strictly partisan impeachment, nonetheless has the numbers from Democrats to approve it. "Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress," Ms Pelosi told her party colleagues. Representatives in the House are expected to vote along party lines, with one Democrat reportedly preparing to switch to the Republican Party to vote against impeachment. The rare undertaking to impeach a president, set to unfold over more than six hours of debate Wednesday, is splitting the lawmakers in Congress much the way Americans have been divided over the issue. Several hundred of protesters march in San Francisco during a demonstration in part of a national impeachment rally Credit: AFP From Alaska to Florida, tens of thousands of Americans marched in support of impeachment Tuesday evening, from a demonstration through a rainy Times Square to handfuls of activists standing vigil in small towns. They carried signs saying "Save the Constitution – Impeach!!!!" and "Criminal-in-Chief." "I really believe that the Constitution is under assault," said one protester, 62-year-old Glenn Conway, of Holly Springs, North Carolina, attending his first political rally in 30 years. "I think we have a president at this point who believes he's above the law." At the heart of the issue is a claim that Mr Trump held back aid to Ukraine in a bid to pressure the country to investigate his political rival Joe Biden and his son Hunter. Mr Biden, the former US vice president, is currently the Democratic front-runner in the race to challenge Mr Trump for the White House in 2020. The trigger for the impeachment investigation was a phone call between Mr Trump and the newly-elected president of Ukraine, Volodymyr Zelenskiy. Concerns about the phone call were first revealed after a whistleblower filed a complaint accusing the president of abusing his power in order to interfere in the 2020 election. The impeachment probe centres on a call between President Zelenskiy and Mr Trump Credit: AFP The whistleblower, a CIA officer previously assigned to the White House, said several officials had raised concerns about Mr Trump's conversation with Mr Zelenskiy. In response, the House Speaker Nancy Pelosi announced in late September that the chamber would open a formal impeachment proceeding to investigate the claims. A day after the impeachment investigation began, the White House released a redacted version of the phone call which revealed that Mr Trump had indeed urged Mr Zelenskiy to do him a "favour" and investigate the Bidens. Overshadowing the request was the fact that it came days after Mr Trump held back almost $400 million in military aid that Congress had approved for Ukraine to defend itself against Russia. Mr Trump has insisted he did nothing wrong in calling for the investigation, saying there was no "quid pro quo" in his dealings with Ukraine. However Democrats have said Mr Trump's actions amount to an attempt to improperly interfere in the 2020 presidential election. The House impeachment resolution says that Trump abused the power of his office and then tried to obstruct the investigation in Congress like "no other" president in history. Mr Trump "betrayed the Nation by abusing his high office to enlist a foreign power in corrupting democratic elections," it states. "President Trump, by such conduct, has demonstrated that he will remain a threat to national security and the Constitution if allowed to remain in office." Only two other presidents have been impeached. Andrew Johnson, who took over the presidency after Abraham Lincoln’s assassination, was impeached in 1868 after being seen as too sympathetic to the defeated Confederacy. Bill Clinton was impeached after lying under oath about his affair with White House intern Monica Lewinsky in 1998. Richard Nixon, who faced an impeachment push, resigned before a vote was held.