All the possible witnesses who might testify in the Senate impeachment trial

Senate Majority Leader Mitch McConnell won't commit to allowing testimony from witnesses in a Senate trial over the impeachment of President Trump. House Speaker Nancy Pelosi won't send over the articles of impeachment. The whole process is at a standstill as they fight over the four people who would testify in the Senate impeachment trial — if Democrats got their way.

In mid-December, the House approved articles of impeachment against Trump for abuse of power and obstruction of Congress. According to the approved articles, Trump "solicited the interference of a foreign government, Ukraine" in a U.S. election and "conditioned two official acts" (a White House meeting with the Ukrainian president, and the disbursal of congressionally appropriated military aid) on Ukraine's compliance with his requests. Further, he "directed the unprecedented, categorical, and indiscriminate defiance of subpoenas."

All of this is in regards to the allegations that Trump pressured the Ukrainian government to investigate former Vice President Joe Biden, who is currently the frontrunner in the 2020 Democratic primary, and his family members. The president also allegedly directed the aid to be withheld from Ukraine until his request was completed.

On Tuesday, McConnell indicated that he had enough Republican support to go forward with the trial without calling additional witnesses. But until Pelosi transmits the articles of impeachment to the Senate, uncertainty will reign. Democratic senators are holding out in hopes of hearing from a few key witnesses who did not testify in the House impeachment hearings — here's who they're focused on.

So, who would testify?

In a December letter to McConnell, Senate Minority Leader Chuck Schumer requested that Chief Justice John Roberts, who will preside over the impeachment trial, subpoena John Bolton, former national security adviser; Mick Mulvaney, acting White House chief of staff; Robert Blair, Mulvaney's senior adviser; and Michael Duffey, the associate director of national security programs at the Office of Management and Budget. Schumer said the four potential witnesses have "direct knowledge of administration decisions regarding the delay in security assistance funds to the government of Ukraine and the requests for certain investigations to be announced by the government of Ukraine."

While McConnell has thus far indicated that he may not allow for any witnesses at all, Bolton recently announced that he's ready to speak, if asked. In a statement on his PAC's website, Bolton wrote that after careful consideration, he has "concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."

Mulvaney, Blair, and Duffey were all subpoenaed in the House impeachment hearings, according to CNN. None of them testified.

Why do Democrats want these witnesses to testify?

As Trump's former national security adviser, Bolton knows a lot about what transpired regarding Ukraine, his lawyer told the House of Representatives's general counsel in a November letter. Bolton "was personally involved in many of the events, meetings, and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far," his lawyer wrote.

Meanwhile, Mulvaney, Trump's acting chief of staff, publicly admitted that there was a quid pro quo with Ukraine and that "we held up the money" to pressure the country. "Get over it," he added. "We do that all the time with foreign policy." The money he was referring to was hundreds of millions of dollars in security aid to Ukraine, intended to protect that country against Russian aggression. Mulvaney then denied ever saying that and claimed there was no quid pro quo.

Mulvaney's senior adviser, Blair, was actually on the phone call that Trump had with Ukrainian President Volodymyr Zelensky, and could provide important insight on what that conversation included, while over at the OMB, Duffey was reportedly directly involved in telling the Department of Defense to hold off on sending the aid to Ukraine, The Washington Post reported.

Would anyone else testify?

Rudy Giuliani, Trump's personal lawyer, has said he would testify if asked. He's been traveling to Ukraine to meet with various people in hopes of finding evidence of corruption in the Biden family. Giuliani claims that Ukraine was working for Democrats in 2016 (a theory Trump's former homeland security adviser said was "completely debunked") and that Biden advocated for the firing of a Ukrainian prosecutor so his son, Hunter, would not be investigated. (As The New York Times has reported, Biden did advocate for a prosecutor's firing, but that was in concert with international consensus at the time and there is no evidence it was to help his son).

"We have found multiple crimes the Bidens have committed: extortion, bribery, and money laundering," Giuliani claimed in an interview on the conservative One America News Network. But as Vox reported, these are conspiracy theories with no evidence to back them up. Even Republican Sen. Lindsey Graham (S.C.) said, "My advice to Giuliani would be to share what he got from Ukraine with the [intelligence community] to make sure it’s not Russia propaganda."

And of course, there are questions regarding whether or not Biden himself would testify, seeing as he and a member of his family are central to this case.

Biden initially said he would not testify, as it would give Trump exactly what he wanted by taking the nation's focus off of the reasons Trump was impeached. But then he backtracked, saying he would in fact obey a subpoena and "honor whatever the Congress in fact legitimately asked me to do."

"But I am just not going to pretend that there is any legal basis for Republican subpoenas for my testimony in the impeachment trial," Biden wrote in a tweet. "That is the point I was making yesterday and I reiterate: this impeachment is about Trump’s conduct, not mine."