Yet many are refusing to break with Trump.
A third line of questioning may lead to another defence - that what took place between the Trump administration and Ukraine, while distasteful, doesn't rise to the level of "high crimes and misdemeanours", the constitutional standard for impeachment. Joe Biden was vice-president for over two years when Hunter served on the board of directors of Burisma, a Ukrainian gas company.
The testimony came from Laura K. Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, who was deposed under oath last month during a session that was disrupted when House Republicans stormed the secure facility where it was being held.
Cooper told investigators that, in a series of July meetings at the White House, she came to understand that Trump's acting chief of staff, Mick Mulvaney, was holding up the military aid for the USA ally. The former House Armed Services Committee chairman and steadfast critic of Russian Federation on Sunday called Trump's use of foreign aid to elicit a political probe from that nation "inappropriate" but accused Democrats of running a "tainted" and "one-sided" probe.
"We have enough evidence from the depositions that we've done to warrant bringing this forward, evidence of an extortion scheme, using taxpayer dollars to ask a foreign government to investigate the president's opponent", Rep. Swalwell said on CBS' Face the Nation.
The most important week so far of the impeachment inquiry is here. Democrats recognize that the onus is on them to make the case to independent voters who don't have time to sift through thousands of pages of transcripts alleging presidential misconduct.
Cooper said she became aware in mid-July that the Office of Management and Budget had frozen Ukraine's security assistance, learning about it initially from National Security Council officials. So far, however, the opinion poll numbers favoring impeachment have not convinced even a single Republican in Congress that Trump's actions are impeachable. Protesters shouted during his talk "lock him up!" and began whistling while he spoke. Reporters also noted that signs spelling out the words "IMPEACH" and "DUMP TRUMP" were posted in the windows of a building overlooking the park.
The contents of Trump's call with Zelensky were revealed in a whistleblower complaint by an anonymous intelligence official.
Trump has made several demands for how he wants GOP lawmakers to defend him.
Tuesday - The White House may release the transcript of an April call between Trump and Ukrainian President Volodymyr Zelensky.
"The call to the Ukrainian President was PERFECT", Trump wrote on Twitter. 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. No, it is much stronger than that. Trump is under threat of becoming only the third president in U.S. history to be impeached - formally charged with violating his duties as president or committing crimes, and placed on trial in the Senate. "Barr has largely been absent from the spotlight since the impeachment inquiry was opened". He's also linked Trump's chief of staff Mick Mulvaney to efforts to coerce Ukraine into a politically motivated investigation.
Trump denies wrongdoing and says he is the victim of a witch hunt by Democrats.
Mr Hurd said, however, he "would love to hear from Mr Hunter Biden" and accused Democrats of running a "partisan exercise".
"I was part of America's Ukraine policy".
It gets to the heart of an allegation of a quid pro quo - alleging that Trump ordered that vital military funding and the promise of a meeting with Zelensky be withheld unless Zelensky announced an investigation into the Bidens.
"Well, I don't know if it was necessarily investigate the Democrats, right?"
The statement adds Croft testified that Ukrainian officials "approached her quietly about the hold on security assistance in the July or August timeframe". Presiding over the trial would likely be Supreme Court Chief Justice John Roberts, who was appointed to the court by Republican President George W Bush in 2005. "It is troubling. It is deeply troubling". This, too, was a break from the other US agencies.
"There was no heavy demand insisting that something had to happen. No, because the aid flowed".