Congress formally opened a new, public phase of its corruption investigation into Donald Trump on Thursday as US lawmakers voted for the first time to advance the impeachment process targeting the US president."Today the House takes the next step forward as we establish the procedures for open hearings... so that the public can see the facts for themselves," House Speaker Nancy Pelosi said."What is at stake in all of this is nothing less than our democracy."The top Democrat and Trump's chief nemesis in Congress addressed fellow lawmakers shortly before her chamber took a Halloween morning vote along party lines - 232 to 196 - to pass a resolution that lays out rules for the next stages of the impeachment process.Trump has repeatedly branded the process as illegitimate and politically motivated and, true to form, his reaction was swift."The Greatest Witch Hunt In American History!" he boomed on Twitter.He called on Republicans to rally behind him even as he faced the embarrassing likelihood of becoming the third president in history to be impeached and placed on trial for removal in the Senate, over an alleged extortion scheme to obtain Ukraine's help to get him reelected in 2020.The embattled president retweeted a rallying cry from Fox News host Laura Ingraham that called on Republicans to "stand together and defend the leader of their party against these smears.""This farce should never be allowed to roll into a winter show trial," read the tweet - suggesting Trump would want the Republican leadership in the Senate to shut the process down.Holding Trump 'accountable'Trump is accused of withholding military aid to compel Ukraine to mount an embarrassing corruption probe against his Democratic election rival Joe Biden - effectively using US foreign policy in an illegal shakedown for his personal political benefit.Trump and loyal Republicans dismiss the case as a "sham," but congressional investigators have heard a steady flow of corroborating evidence from government officials testifying behind closed doors on Capitol Hill.The landmark resolution did not have unanimous Democratic approval. Two Democrats joined all Republicans in opposing the measure, although former Republican Justin Amash, now an independent, supported it.The inquiry now moves into the public eye - giving Americans the chance to hear on live television the evidence against Trump.The House Intelligence Committee, which has led the inquiry so far, will host open hearings, presenting witnesses and documentary evidence and allowing Republicans to challenge the case against Trump.The minority can also suggest subpoenas but majority Democrats have final say, a rule that has angered Republicans."Trying to put a ribbon on a sham process doesn't make it any less of a sham," House Republican Jim Jordan fumed.Democrats argue that the vote neutralises a key Republican talking point that the inquiry has no validity because the full House did not sign on. Neither the House rules nor the US Constitution require such a vote to start an impeachment inquiry.If the case against Trump is deemed strong enough, the House Judiciary Committee will draw up formal charges against the president - articles of impeachment - to be voted on by the full House.That process could be completed by year end. The Democrat-controlled House is expected to approve impeachment, which would then see Trump go on trial for removal in the Senate, where Republicans have a majority.Democrats dismissed the charges that the process is partisan."There is serious evidence that President Trump may have violated the Constitution," said Jim McGovern, Chairman of the House Rules Committee which drew up the resolution."If we don't hold this president accountable, we could be ceding our ability to hold any president accountable."Witnesses back allegationsNearly a dozen witnesses so far have confirmed in House depositions the accusations that, in a concerted effort with top aides and his personal lawyer, Trump pressured Ukraine to help his reelection effort in 2020 by producing dirt on Biden, the former vice president.The allegations focus on a July 25 phone call in which Trump pressed Ukraine President Volodymyr Zelensky to open investigations into Biden and Biden's son, who worked with a Ukraine energy firm.Tim Morrison, the White House National Security Council's top Russia expert, arrived on Capitol Hill Thursday to be deposed.According to other witnesses, Morrison, who resigned late Wednesday, has personal knowledge of the White House effort to freeze military aid to Ukraine to pressure Zelensky to launch the political investigations.Investigators also have called on Trump's former national security advisor John Bolton to testify, along with two White House national security lawyers.Bolton, other witnesses have said, disagreed strongly with Trump's tactics toward Ukraine and the involvement of his personal lawyer Rudy Giuliani in Ukraine policy.