WASHINGTON, Dec 5 (Reuters) - U.S. House of Representatives Speaker Nancy Pelosi on Thursday said she has directed a House committee to draft articles of impeachment against President Donald Trump over his effort to pressure Ukraine to investigate a political rival, a historic step that sets up a fight over whether to oust him from office.Pelosi, speaking in somber tones in a televised statement, accused Trump of abusing his power and alluded to Britain's King George III, the monarch against whom the American colonies rebelled in forming the United States in 1776. "Our democracy is what is at stake. The president leaves us no choice but to act because he is trying to corrupt, once again, the election for his own benefit. The president has engaged in abuse of power, undermining our national security and jeopardizing the integrity of our elections," Pelosi said.The impeachment fight undertaken by Pelosi and her fellow House Democrats is unfolding even as the Republican president is running for re-election in 2020.At the heart of the battle is Trump's request in July that Ukraine launch an investigation targeting former U.S. Vice President Joe Biden, a top contender for the 2020 Democratic presidential nomination to face Trump."Sadly, but with confidence and humility, with allegiance to our founders and our heart full of love for America, today I am asking our chairman to proceed with articles of impeachment," Pelosi said, referring to House Judiciary Committee Chairman Jerrold Nadler.Articles of impeachment represent formal charges against Trump and would originate in the Judiciary Committee before going to the full House. If the Democratic-led House passes articles of impeachment as expected, that would lead to a trial in the Senate on whether to convict Trump of those charges and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.Trump reacted to Pelosi's announcement on Twitter by writing, "The Do Nothing, Radical Left Democrats have just announced that they are going to seek to Impeach me over NOTHING.""The good thing is that the Republicans have NEVER been more united. We will win!" Trump added, while predicting that impeachment now "will be used routinely to attack future Presidents. That is not what our Founders had in mind."Pelosi acted after receiving overwhelming support to push forward with the impeachment charges in a meeting with House Democrats on Wednesday night, a source familiar with the meeting said.On Wednesday, the Judiciary Committee held a hearing in which three constitutional law experts called by Democratic lawmakers said Trump had engaged in conduct that represents impeachable offenses under the Constitution. A fourth expert called by Republican lawmakers called the Democratic-led impeachment inquiry rushed and flawed.The three law professors chosen by the Democrats made clear that they believed Trump's actions constituted impeachable offenses including abuse of power, bribery, obstruction of Congress and obstruction of justice.The focus of the inquiry is the July 25 telephone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to open an investigation into Biden and his son Hunter Biden and into a discredited theory promoted by Trump and his allies that Ukraine, not Russia, meddled in the 2016 U.S. election.Hunter Biden had joined the board of Ukrainian energy company Burisma while his father was U.S. vice president. Trump has accused the Bidens of corruption without offering evidence. They have denied wrongdoing.The House Judiciary Committee will hold a hearing on Monday to receive presentations on evidence from lawyers representing the two parties in the impeachment inquiry, Nadler said.The House Intelligence Committee this week submitted findings from its inquiry into Trump's dealings with Ukraine.Democrats have accused Trump of abusing his power by withholding $391 million in security aid to Ukraine - a vulnerable U.S. ally facing Russian aggression - as leverage to pressure Kiev into conducting investigations politically beneficial to Trump. The money - approved by the U.S. Congress to help Ukraine combat Russia-backed separatists in the eastern part of the country - was provided to Ukraine in September after the controversy had spilled into public view.'A KING-PRESIDENT'"When crafting the Constitution, the founders feared the return of a monarchy in America. And having just fought a War of Independence, they specifically feared the prospect of a king-president corrupted by foreign influence," Pelosi said.Trump has denied any wrongdoing and called the impeachment investigation a hoax.The House may vote by year's end on the formal impeachment charges, but Democrats, who control the chamber, say no decision has been made at this point on the specific charges. Those could include abuse of power, bribery, obstruction of Congress and obstruction of justice.Judiciary Committee Democrats on Wednesday said they may look beyond Trump's relations with Ukraine to include Trump's earlier efforts to impede former Special Counsel Robert Mueller's investigation that documented Russian interference in the 2016 election and extensive contacts between Trump's campaign and Moscow. They stopped short of saying that could trigger a separate charge.No president has ever been removed from office through impeachment, though Republican Richard Nixon resigned in 1974 after the House began the impeachment process in the Watergate corruption scandal. Two other presidents were impeached by the House but acquitted by the Senate.The last impeachment effort in the United States came in 1998 when a Republican-led House passed articles of impeachment against Democratic President Bill Clinton - charges arising from a sexual relationship he had with a White House intern. The Senate then acquitted Clinton in 1999, leaving him in office.The other president who was impeached was Andrew Johnson. He was impeached by the House in 1868, three years after the end of the U.S. Civil War, but left in office by the Senate.Trump's fellow Republicans in both chambers have stood by him, and have accused Democrats of seeking to overturn the 2016 election. Republicans in the Senate have given no signs they would break with the president now.Polling has shown Americans are also largely divided along party lines over impeachment.