Democrats laid out a formal roadmap for President Donald Trump's impeachment Thursday as they accumulated more evidence to support charges that he improperly pushed Ukraine to boost his own 2020 electoral prospects.  

One day after a decorated army officer told Congressional investigators he witnessed Trump and a senior diplomat pressure Ukraine, three other State Department officials on Wednesday offered more evidence in testimony that supported the allegations against the U.S. leader. 

And the inquiry testimony set dates for three more witnesses, including Trump's estranged former National Security Advisor John Bolton, who would have had first-hand knowledge of the president's alleged effort to leverage military aid to Ukraine in exchange for President Volodymyr Zelensky investigating his Democratic rival Joe Biden. 

Even before those interviews take place, Democrats proposed rules to formalize the investigation and set its next phase – which would have open evidentiary hearings that Trump or his lawyers could take part in – ahead of drawing up articles of impeachment.  

On Wednesday, the House Rules Committee agreed by a party line vote to put the resolution up for approval before the full House of Representatives on Thursday.  

"I didn't run (for office) to impeach the president," House Rules Committee Chairman James McGovern said after the vote. "These are very, very serious matters, and we need a process in place so people know how we're going to proceed. I think this is a good transparent process."  

Earlier in the day, McGovern said, "There is serious evidence that the president may have violated the constitution."  

Trump: Probe is a 'witch hunt' 

Trump blasted the investigation as a "witch hunt" and again claimed there was no "quid pro quo" in his dealings with Zelensky. 

But Democrats said the evidence was only getting stronger that he did push Ukraine to open investigations into Biden, whose son had ties to a powerful Ukraine energy company.  

The impeachment rules will formalize a process Republicans have alleged has no official grounds, but the Democrats' comfortable majority in the House is likely to ensure its passage.  

"The evidence we have already collected paints the picture of a President who abused his power by using multiple levers of government to press a foreign country to interfere in the 2020 election," the leaders of the impeachment inquiry said late Tuesday.  

"Following in the footsteps of previous impeachment inquiries, the next phase will move from closed depositions to open hearings where the American people will learn firsthand about the President's misconduct." 

Aid hold 'directed by Trump' 

On Wednesday, State Department diplomat Christopher Anderson, a former aide to the U.S. special representative for Ukraine Kurt Volker, told the inquiry Trump's lawyer Rudy Giuliani injected himself into Ukrainian policy discussions to demand Kiev open investigations that could aid Trump. 

Another former Volker aide, Catherine Croft, said in prepared testimony that Trump's chief of staff Mick Mulvaney ordered military aid to Ukraine frozen days before a July 25 phone call in which Trump pressured Zelensky to investigate Biden.  

"The only reason given was that the order came at the direction of the president," Croft said. 

And in separate testimony, John Sullivan, nominated to be U.S. ambassador to Russia, told a Congressional hearing that Giuliani was part of an effort designed to smear then-ambassador to Ukraine Marie Yovanovitch, who had reportedly resisted White House efforts to pressure Kiev to investigate Biden.  

"Soliciting investigations into a domestic political opponent, I don't think that would be in accord with our values," Sullivan told the Senate Foreign Relations Committee.