A Democratic senator questioned why the House of Representatives, in its monthslong impeachment inquiry, didn't fight harder to have key witnesses testify before voting to adopt articles of impeachment against President Donald Trump, according to Roll Call.

As impeachment hits a stalemate over Senate trial rules and potential witnesses, West Virginia Democratic Sen. Joe Manchin said the House made things more difficult by not going to court in order to force people like White House chief of staff Mick Mulvaney and former national security adviser John Bolton to testify.

"I don't know why the House ... didn't go to court. You know, why they didn't go after these witnesses that are so pertinent to the whole accusation. That makes it very difficult," Manchin said. "Hindsight's 20/20. I can't tell you what was in their mind, why they did what they did, but they had the ability to go to court, subpoena these witnesses and make sure they testify, either in closed — in a closed deposition or whatever. Bolton had firsthand information. Mulvaney has firsthand information, and there's a lot of people on the front lines."

House Democrats made the decision to proceed with a quicker impeachment inquiry process instead of engaging in the potentially lengthy legal battle that would've been required to force Mulvaney and Bolton to comply with subpoenas to testify. As a result, most witnesses did not have direct conversations with Trump about his Ukraine dealings.

The one witness who had spoken directly to Trump, U.S. Ambassador to the European Union Gordon Sondland, was a relatively unreliable witness due to his spotty memory and lack of contemporaneous documentation of conversations and events.

Mulvaney would be able to speak directly to the details behind the delay in military aid to Ukraine, as he is also the head of the Office of Management and Budget, in addition to his role as acting chief of staff. His comments at a news conference that linked the aid to investigations Trump requested of Ukraine into the 2016 election and the Bidens have been used against Trump.

Bolton's knowledge and perspective on the Ukraine saga is somewhat unclear, but his close involvement is undeniable, as many of the witnesses who did testify spoke about Bolton's apparent discomfort with the situation. Bolton himself has hinted at key knowledge, but did not volunteer testimony.

Now, House Speaker Nancy Pelosi is threatening to withhold articles of impeachment from the Senate until the two sides reach an agreement on how the trial will go — including Democrats' ability to call Mulvaney and Bolton as witnesses.