Sen. Chris Van Hollen, D-Md., said Republican Sen. Lisa Murkowski put her finger on the big issue of an impeachment trial when she expressed concern about the Senate leadership coordinating with the White House on the trial.

"Is Sen. Mitch McConnell, the Republican leader, going to try to rig this trial, working in lockstep with the president and his lawyers? Or is he going to allow a fair trial?" Van Hollen said on ABC's "This Week" Sunday. "We keep hearing President Trump say he's going to be exonerated. Look, if you have a rigged trial there's no exoneration in acquittal."

President Donald Trump became the third president in U.S. history to be impeached, but House Speaker Nancy Pelosi has not yet delivered the articles of impeachment to the Senate.

Pelosi has said that she will not transmit the articles until she sees "the process that is set forth in the Senate."

Senate Majority Leader McConnell and Senate Minority Leader Chuck Schumer have been at an impasse over the structure of the trial, namely whether the Senate would call witnesses.

"The witnesses and documents are the main arguments here," Schumer said at a press conference on Monday. "We must decide them and hopefully we can decide them in a bipartisan way before going forward with the trial."

In an interview with Fox News, McConnell said, "Everything I do during this, I'm coordinating with White House counsel. There'll be no difference between the president's position and our position as to how to handle this."

Sen. Lisa Murkowski, R-Alaska, said on Tuesday she was "disturbed" by McConnell's saying that he would work in coordination with the White House on an impeachment trial.

"When I heard that I was disturbed," she told Alaska NBC affiliate KTUU. "To me it means that we have to take that step back from being hand in glove with the defense, and so when I heard what Leader McConnell has said, I happened to think that further confused the process."

Van Hollen has been supportive of Pelosi's decision to hold off on sending articles of impeachment to the Senate, telling MSNBC earlier this week that the speaker was "exactly right."

He also said he thought the house has made the "overwhelming case for impeachment" in an interview with CNN on Monday.

This is a developing story. Please check back for updates.

ABC News' Mariam Khan contributed to this report.