The US House of Representatives launched momentous televised impeachment hearings on Wednesday as Democrats seek to make the case to the American public that President Donald Trump abused the powers of his office."There are few actions as consequential as the impeachment of a president," said House Intelligence Committee chairman Adam Schiff, the California congressman overseeing the historic inquiry in the Democratic-controlled House."The questions presented by this impeachment inquiry are whether President Trump sought to exploit (Ukraine's) vulnerability and invite Ukraine's interference in our elections," Schiff said. "If this is not impeachable conduct, what is?"The 73-year-old Trump faces the most perilous challenge of his tumultuous three-year tenure in the White House as the public impeachment hearings began under the glare of live television cameras.Speaking minutes before the start of the hearings, Nancy Pelosi, the Democratic speaker of the House, said the probe was necessary to show Trump he can't do "whatever he wants.""That he is not above the law," Pelosi said. "And that he will be held accountable." Democrats who control the House plan to prove over several weeks of hearings that Trump abused his office by asking Ukraine to conduct a politically motivated investigation into his potential 2020 Democratic president rival Joe Biden.Trump, who maintains he did nothing wrong, lashed out at the inquiry with a series of tweets early Wednesday, morning citing prominent supporters who called it a "partisan sham."Addressing the intelligence committee hearing, the panel's top Republican Devin Nunes assailed the impeachment process as "a carefully orchestrated media smear campaign" that was part of a "scorched-earth war against President Trump."The investigation threatens to make Trump only the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998, although to be removed from office he would need to be convicted by the Republican-led Senate.Neither Johnson or Clinton was convicted and removed. But in 1974 Richard Nixon resigned in the face of certain impeachment and removal from office for the Watergate scandal.Fiery hearings expected Hearings are expected to be fiery as a series of government officials take the stand to testify about Trump's Ukraine machinations during the middle of this year.Coming just one year before national elections, the hearings carry great risks for both parties and no certain reward, with a divided US electorate weary of Washington infighting.Polls show a slim majority of Americans favor impeaching the president. But they also show that Trump's sizable voter base, which delivered his shock victory in 2016, rejects the allegations. Trump has focused his personal defense on ensuring Republicans in Congress heed their views.Republicans accuse the soft-spoken and prosecutorial Schiff of an unfair and unconstitutional process.They have also sought, in closed door depositions over the last six weeks, to refocus attention on Biden's link, through his son, to Ukraine, and on the widely discredited theory Trump apparently believes that Ukraine assisted Democrats in the 2016 election.But Schiff has said he will not put up with attempts to hijack the hearings and turn them into a political circus.Democrats have amassed evidence that Trump sought to leverage Ukraine President Volodymyr Zelensky's desire for a meeting between the two leaders and for some $391 million in aid to get Ukraine to find dirt on Biden, who could face Trump in next year's presidential election.The key evidence is the official White House transcript of a July 25 phone call showing Trump pressuring Zelensky to open investigations into Biden and the 2016 conspiracy theory.The White House has refused to hand over other records on Ukraine policy or allow top Trump aides involved in the decision to pressure Zelensky to testify.'Politically motivated prosecutions' On Tuesday Trump's chief of staff Mick Mulvaney -- who has publicly confirmed the broad outlines of Democrats' allegations - rejected a subpoena to appear before the committee.The first witnesses Wednesday will be William Taylor, the top US diplomat in Ukraine, and George Kent, Deputy Assistant Secretary of State for European and Eurasian AffairsBoth have already testified in private that Trump clearly used his power and aid to pressure Zelensky for investigations that would help him in the 2020 vote."I had concerns that there was an effort to initiate politically motivated prosecutions that were injurious to the rule of law, both in Ukraine and the US," Kent told investigators.On Friday, Marie Yovanovitch, the US ambassador to Ukraine whom Trump removed earlier this year, will testify.Democrats on Tuesday unveiled the schedule for public testimony next week by eight more witnesses, all of whom previously testified behind closed doors.House Republicans are preparing to argue that Trump was within his rights, given Ukraine's history of deep corruption."Democrats want to impeach President Trump because unelected and anonymous bureaucrats disagreed with the President's decisions," they said in a strategy memorandum over the weekend.