The top Senate Democrat on Sunday called for subpoenaing several senior Trump administration officials who have yet to testify in the House’s impeachment probe as witnesses for President Trump’s likely trial — part of an opening salvo in negotiations that could determine the parameters for the Senate proceedings next month.

In a letter to Senate Majority Leader Mitch McConnell (R-Ky.), Senate Minority Leader Charles E. Schumer (D-N.Y.) outlined a number of procedural demands that Democrats say would make the Senate trial fair and able to be completed “within a reasonable period of time.”

That includes subpoenas issued by Chief Justice John G. Roberts Jr. for acting White House chief of staff Mick Mulvaney; Robert Blair, a senior adviser to Mulvaney; former national security adviser John Bolton; and Michael Duffey, a top official at the Office of Management and Budget. Mulvaney, Blair and Duffey had been subpoenaed by the House committees and defied the summons; Bolton has not been subpoenaed but indicated he would fight one in court.

“The trial must be one that not only hears all of the evidence and adjudicates the case fairly; it must also pass the fairness test with the American people,” Schumer wrote to McConnell in the letter sent Sunday. “That is the great challenge for the Senate in the coming weeks.”

Under Schumer’s proposal, the trial proceedings would begin Jan. 6, although Roberts and the senators wouldn’t be sworn in until Jan. 7, and House impeachment managers would begin their presentations on Jan. 9.

The proposal on witnesses is almost certain to draw the most resistance from McConnell. The top Senate Republican, as well as a growing consensus of GOP senators, would prefer not to call the type of high-wattage witnesses that Trump has demanded — such as Hunter Biden, son of former vice president Joe Biden, and the whistleblower whose complaint triggered the impeachment inquiry — and McConnell has warned privately that a battle over witnesses would be “mutually assured destruction.”

Schumer proposes allowing up to eight hours of testimony per witness, and the minority leader is also calling for documents that “we believe will shed additional light on the administration’s decision-making” on the reasons behind the holdup of nearly $400 million in military assistance to Ukraine that has become a focal point of the House investigation.

“Conducting the trial according to this plan will also allow the public to have confidence in the process and will demonstrate that the Senate can put aside partisan concerns and fulfill its constitutional duty,” Schumer wrote in the letter.

Doug Andres, a spokesman for McConnell, said in response to the letter that the majority leader “has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon. That timeline has not changed.”

The House is on track to vote to impeach Trump on Wednesday, sending two articles of impeachment for abuse of power and obstruction of Congress to the Senate. Under the Constitution, the Senate would hold a trial with all 100 senators as jurors. Two-thirds of those present would be required for a conviction.

McConnell and Schumer could begin hashing out the scope of a Senate trial as early as this week, and Schumer’s proposal came on a day when Democrats ramped up their criticism of McConnell and his assertion that he was in “total coordination” with the White House on Trump’s trial despite his role as a juror.

GOP senators defended their right to work for Trump’s acquittal. But McConnell’s statement, according to House Judiciary Committee Chairman Jerrold Nadler (D-N.Y.), was akin to “the foreman of the jury saying he’s going to work hand in glove with the defense attorney.”

“That’s in violation of the oath that they’re about to take, and it’s a complete subversion of the constitutional scheme,” Nadler said on ABC’s “This Week.”

Senators take an oath to “do impartial justice” at the start of any impeachment trial — but several Republican senators argued that impartiality doesn’t cover politics.

“I am clearly made up my mind. I’m not trying to hide the fact that I have disdain for the accusations in the process,” Sen. Lindsey O. Graham (R-S.C.) said Sunday on CBS News’s “Face the Nation.”

“Senators are not required, like jurors in a criminal trial, to be sequestered, not to talk to anyone, not to coordinate. There’s no prohibition,” Sen. Ted Cruz (R-Tex.) said on “This Week,” calling impeachment “inherently a political exercise” and Trump’s impeachment a “partisan show trial.”

In his letter, Schumer also pitched several procedural rules to guide the Senate proceedings, many mirroring the parameters from former president Bill Clinton’s 1999 impeachment trial. For instance, Schumer is calling for the House impeachment managers and Trump’s defense team to have 24 hours each in the Senate to make their case and 16 hours for senators to ask questions of either side.

The proposal also includes six hours total for final arguments and a maximum of 24 hours for senators to deliberate before they render a verdict on whether to convict the president.

Senate GOP leaders have indicated they want to limit the trial to a short proceeding and omit witnesses. That isn’t sitting well with House Democratic leaders, who say senators should use their trial to secure evidence and testimony the White House prevented House investigators from accessing.

“They don’t want the American people to see the facts,” House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) said Sunday on ABC, appearing alongside Nadler.

There are some Senate Republicans who want to hear from witnesses at the trial, particularly if Trump continues to insist on them. In addition to Hunter Biden and the whistleblower, Trump has called on Schiff and House Speaker Nancy Pelosi (D-Calif.) to appear as witnesses.

“You can be sure we’re going to allow the president to defend himself,” Cruz said, adding: “That means, I believe, if the president wants to call witnesses, if the president wants to call Hunter Biden or wants to call the whistleblower, the Senate should allow the president to do so.”

Hunter Biden sat on the board of Ukrainian energy company Burisma for five years and was paid as much as $50,000 a month, despite having no expertise on the subject matter. As Democrats have made the case that Trump tried to use his office to pressure the Ukrainian leader into announcing investigations against a political rival, several Republicans have rallied around the countercharge that Trump was right to be concerned about “corruption” involving the Bidens — though no evidence has surfaced that Joe Biden, who was closely involved in U.S. policy toward Ukraine, made any decisions to advantage the company.

“I love Joe Biden, but none of us are above scrutiny,” Graham said Sunday.

He added that the Senate could look at all of those issues — as well as whatever new information Trump’s lawyer Rudolph W. Giuliani unearthed in his latest trip to Ukraine — “after impeachment.”

On Sunday, Giuliani tweeted a string of vague and unsubstantiated allegations about what he called “Obama-era corruption” in Ukraine — including a charge that Viktor Shokin, the Ukrainian prosecutor general who was ousted on suspicion of rampant corruption, “was poisoned, died twice, and was revived.”

It is not clear whether the Senate will be forced to hold separate votes on witnesses — or whether most of the GOP would hold rank in that situation. It takes 51 senators to approve a motion.

There are 53 Republicans in the Senate, meaning the GOP can afford to lose no more than two senators on any motion for McConnell to fully control the course of the trial.