Both Taylor and George Kent testified about their concerns about pressure by Trump and allies to get Ukraine to investigate Democratic political rival Joe Biden in a dramatic hearing that pitted Democratic and Republican lawmakers against each other.
An important revelation came from Taylor, acting ambassador to Ukraine, who pointed to the Republican president's keen interest in getting Ukraine to investigate Biden, a former vice president, and reiterated his understanding that $391 million in US security aid was withheld from Kiev unless it cooperated.
Taylor said a member of his staff overheard a July 26 phone call between Trump and Gordon Sondland, a former political donor appointed as a senior diplomat, in which the Republican president asked about those investigations and Sondland told him that the Ukrainians were ready to proceed.
Following the call - which occurred a day after Trump had asked Ukraine's president during a phone call to conduct these investigations - the staff member asked Sondland, the US ambassador to the European Union, what Trump thought about Ukraine, Taylor said.
"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Taylor testified, referring to Trump's personal lawyer Rudy Giuliani.
Asked by Adam Schiff, the committee's Democratic chairman, if that meant Trump cared more about the investigations than about Ukraine, Taylor said, "Yes, sir."
Republican lawmakers called Taylor's account hearsay and noted that Ukraine's president has not said he felt pressured by Trump.
With a potential television audience of tens of millions looking on, Schiff opened the historic session - the first impeachment drama in two decades - in an ornate hearing room packed with journalists, lawmakers and members of the public.
Schiff's accusations that Trump abused his power was met by a staunch denial by the panel's senior Republican, Devin Nunes, of the Republican president's complicity in a saga that revolves around whether Trump and his aides improperly pressured Ukraine to dig up dirt on Biden, and a leading contender for the Democratic nomination for the 2020 election, for his political benefit.
Taylor and Kent, the deputy assistant secretary of state for European and Eurasian affairs, expressed concern that US security aid - as well as a meeting with Trump - was withheld from Ukraine as leverage over Kiev.
"The questions presented by this impeachment inquiry are whether President Trump sought to exploit that ally's vulnerability and invite Ukraine's interference in our elections," Schiff said in his opening statement.
"Our answer to these questions will affect not only the future of this presidency, but the future of the presidency itself, and what kind of conduct or misconduct the American people may come to expect from their commander-in-chief," Schiff said.
Schiff added, "If this is not impeachable conduct, what is?"
This week's sessions, where Americans are hearing directly for the first time from people involved in events that sparked the congressional inquiry, may pave the way for the Democratic-led House to approve articles of impeachment - formal charges - against Trump.
That would lead to a trial in the Senate on whether to convict Trump of those charges and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.
Asked about the impeachment proceedings, Trump, at an Oval Office meeting with visiting Turkish President Tayyip Erdogan, told reporters he was "too busy to watch" and again called it a "a witch hunt, a hoax."
Nunes accused the Democrats of conducting a "carefully orchestrated smear campaign" using "a horrifically one-sided process" and accused "Democrats, the corrupt media and partisan bureaucrats" of trying to overturn the results of the 2016 election won by Trump.
He hewed to the Republican strategy of arguing that Trump did nothing wrong or impeachable when he asked Ukraine's new president to investigate Biden.
"It's nothing more than an impeachment process in search of a crime," Nunes said.
The focus of the inquiry is on the July 25 telephone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to open a corruption investigation into Biden and his son Hunter Biden and into a discredited theory that Ukraine, not Russia, meddled in the 2016 US election. Hunter Biden had been a board member for a Ukrainian energy company called Burisma.
Democrats are looking into whether Trump abused his power by withholding the security aid to Ukraine - a vulnerable US ally facing Russian aggression - as leverage to pressure Kiev into conducting investigations politically beneficial to Trump. The money - approved by the US Congress to help Ukraine combat Russia-backed separatists in the eastern part of the country - was later provided to Ukraine.
'RULE OF LAW'
The witnesses were up next. Taylor, a career diplomat and former US Army officer, previously served as US ambassador to Ukraine and is now the chargé d'affaires of the US embassy in Kiev. Kent oversees Ukraine policy at the State Department.
"I do not believe the United States should ask other countries to engage in selective, politically associated investigations or prosecutions against opponents of those in power, because such selective actions undermine the rule of law regardless of the country," Kent said.
Taylor said he found two channels of US policy toward Ukraine - one regular and one "highly irregular" - and recounted how a Trump meeting with the Ukrainian president was improperly conditioned on Kiev agreeing to investigate Burisma and the debunked notion of Ukrainian interference in the 2016 election.
Taylor said he became aware that a hold on the security aid was contingent Ukraine opening the investigations and that was most alarming to him because it entailed "security assistance to a country at war."
He said he still believes what he said in a text message that was released during his previous closed-door testimony. "I wrote that withholding security assistance in exchange for help with a domestic political campaign in the United States would be 'crazy.' I believed that then, and I believe it now," Taylor said.
Taylor testified that Trump "had a feeling of having been wronged by the Ukrainians" and the investigations were "something that he thought they owed him."
In the last US impeachment drama, Republicans, who then controlled the House, brought impeachment charges against Democratic President Bill Clinton in a scandal involving his sexual relationship with a White House intern. The Senate ultimately voted to keep Clinton in office.
Only two US presidents ever have been impeached. No president has been removed through the impeachment process.
Trump has denied any wrongdoing and has derided some of the current and former US officials who have appeared before committees as "Never Trumpers," though he has provided no proof that they were opponents.
Kent said he had been alarmed by efforts by Giuliani and others to pressure Ukraine. Kent said Giuliani - who Democrats have accused of conducting a shadow foreign policy effort in Ukraine to benefit the president - had conducted a "campaign full of lies" against Marie Yovanovitch, who was abruptly pulled from her post as US ambassador to Ukraine in May. She will give public testimony on Friday.
Democrats are hoping to convince independent voters and other doubters that Trump was wrong not only in asking Ukraine to dig up dirt on his rival but in making it a "quid pro quo" - a Latin meaning a favor in exchange for a favor.
Nunes accused the Democrats of pursuing impeachment after failing to gain more politically from former Special Counsel Robert Mueller's investigation that detailed Russian interference in the 2016 election to boost Trump's candidacy. Mueller documented extensive contacts between the Trump campaign and Russia but found insufficient evidence to prove a criminal conspiracy.
When Republicans took over questioning, they kept to their strategy of focusing the spotlight on the Bidens, and not Trump's actions, but they failed to elicit any testimony implicating them in wrongdoing.