Congressional Democrats have announced the first public hearings next week in an inquiry that may seek to remove President Donald Trump from office.
Three state department officials will testify first. So far lawmakers from three key House committees have heard from witnesses behind closed doors.
The impeachment inquiry centres on claims that Mr Trump pressured Ukraine to publicly announce an investigation into political rival Joe Biden.
He said: “We are getting an increasing appreciation for just what took place during the course of the last year – and the degree to which the president enlisted whole departments of government in the illicit aim to get Ukraine to dig up dirt on a political opponent.”
Mr Trump has been making discredited corruption claims about former US vice-president Joe Biden, whose son Hunter Biden once worked for a Ukrainian gas company.
It shows Mr Taylor told lawmakers it was his “clear understanding” that the president had withheld nearly $400m (£310m) in US military aid because he wanted Ukraine to investigate the Bidens.
Joe Biden is a Democratic front-runner for the presidential election a year from now.
Also scheduled to testify publicly next Wednesday is career state department official George Kent.
Mr Kent reportedly told lawmakers that department officials had been sidelined as the White House put political appointees in charge of Ukraine policy.
Former US ambassador to Ukraine Marie Yovanovitch, who was recalled in May after falling from favour with the White House, is due to testify on Friday next week.
She told the hearing last month that she had felt threatened by Mr Trump’s remark to Ukraine’s president that was “going to go through some things”.
Quick facts on impeachment
Impeachment is the first part – the charges – of a two-stage political process by which Congress can remove a president from office.
If, following the hearings, the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial.
A Senate vote requires a two-thirds majority to convict and remove the president – unlikely in this case, given that Mr Trump’s party controls the chamber.
Only two US presidents in history – Bill Clinton and Andrew Johnson – have been impeached, but neither was convicted.