US President Donald Trump speaks in front of fast food he purchased for a ceremony honoring the 2018 College Football Playoff National Champion Clemson Tigers, in the State Dining Room of the White House in Washington, DC. Photo: AFP

 
Democrats laid out a formal roadmap for US 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 US 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 firsthand 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 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.