WASHINGTON — Senate Minority Leader Chuck Schumer proposed calling former national security adviser John Bolton and acting White House chief of staff Mick Mulvaney as witnesses at an impeachment trial for President Donald Trump in a letter to Senate Majority Leader Mitch McConnell on Sunday.

The offer is intended as a signal that Democrats are seeking an evidentiary trial, not intending to simply rely on the House investigation.

Schumer, D-N.Y., proposed that the Senate subpoena four people who are close to the president or are expected to know about the delay of about $400 million in military aid to Ukraine: Mulvaney; Bolton; Robert Blair, senior adviser to Mulvaney; and Michael Duffey, associate director for national security at the Office of Management and Budget. Schumer proposed that each side question witnesses for four hours each.

House Democrats previously subpoenaed Mulvaney, Duffey and Blair. Neither they nor Bolton testified in the House proceedings.

Schumer added that Democrats "would of course be open" to testimony of anyone else with "direct knowledge" of aid to Ukraine if House prosecutors or the president's lawyers were to request it.

"There's not a single reason that has been given why the four witnesses we've asked for, why the documents we've asked for, should not be presented," Schumer told MSNBC's "Morning Joe" on Monday. "I don't know what they'll say. Maybe they'll be exculpatory to President Trump. But to not have them is to engage in a cover up. Is to say we're afraid. And the American people will ask of President Trump and people like Mitch McConnell if he doesn’t have a full and fair trial, what are you hiding? What are you hiding?"

Schumer voiced opposition to "extraneous" witnesses, like former Vice President Joe Biden's son Hunter Biden, being called to testify in the impeachment trial, saying specifically of the younger Biden that he has no direct knowledge of the allegations facing Trump.

"I don’t know a single person who said, 'Does Hunter Biden know anything about the specific facts that were presented here?'" Schumer said.

Of the witnesses he seeks to testify, Schumer said, "These facts are yearning to come out." Asked if the House should withhold submitting articles of impeachment to the Senate for now so it can further its investigation, Schumer said it's "premature" for him to make such a judgment and that he's "hopeful we can get a fair trial."

White House press secretary Stephanie Grisham responded to Schumer's morning remarks, mocking him over calling for a fair trial.

"'Let us hope that fairness will prevail' a laughable quote from @SenSchumer this AM....after the dems release an 'impeachment report' in the middle of the night," Grisham tweeted. "Thankfully the people of this country continue to see the partisan sham that this is."

Schumer said Monday afternoon that he had told McConnell a few weeks ago that he was ready to discuss trial rules, but that "instead of talking to me, he spoke publicly about what a trial may look like, and said he was taking his cues from the White House."

"The four witnesses we propose have direct knowledge of why the aid to Ukraine was delayed. We don't know what kind of evidence they will present," he said at a press conference on Capitol Hill. "They might present exculpatory evidence that helps President Trump. It may be incriminating against the president, but they should be heard."

"Trials have witnesses. That's what trials are all about. And documents," he added.

McConnell, R-Ky., has indicated that he hopes he has the support of 51 senators for a speedy resolution. He told Sean Hannity of Fox News last week that he and the president's counsel were working closely and that he hoped that there would not be "a single Republican who votes for either of these articles of impeachment."

Democrats on Sunday pushed back at GOP senators who they said appeared to have pre-judged the proceedings.

"Senate Democrats believe strongly, and I trust Senate Republicans agree, that this trial must be one that is fair, that considers all of the relevant facts, and that exercises the Senate's 'sole Power of Impeachment' under the Constitution with integrity and dignity," Schumer wrote to McConnell. "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 also proposed that the Senate issue subpoenas for electronic communications and memoranda of relevant officials in the White House, the budget office and the State Department in a "narrowly drawn" request that is "neither burdensome or time-consuming."

House Democrats' investigation said security aid for Ukraine and a White House call between Trump and Ukrainian President Volodomyr Zelenskiy were predicated on an announcement by Zelenskiy of an investigation into Biden and his son, who sat on the board of the Ukrainian energy company Burisma.

Schumer and McConnell are expected to meet early this week to discuss how a Senate trial would work. If they reach no agreement, the Senate would vote at each step of the process. Each vote would require the support of 51 senators.

Republicans control 53 seats, but several — including Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah — have indicated they want a full trial.

Schumer proposed that the Senate proceedings begin on Monday, Jan. 6, with senators' being sworn in as jurors and Chief Justice John Roberts' being sworn in as presiding officer ‪the next day.

The House impeachment managers, who would effectively act as prosecuting attorneys, would be recognized ‪on Jan. 9‬ to present their case for a maximum of 24 hours, followed by the president's counsel for the same amount of time.

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Schumer says his proposal is modeled after the resolutions setting up the impeachment trial of President Bill Clinton's in 1999. However, there were two resolutions that year, with the second — which was adopted along party lines — deciding the witnesses and more controversial issues. Schumer has proposed passing one resolution.

Doug Andres, a spokesman for McConnell, would not respond directly to the proposal Sunday. "Leader McConnell has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon. That timeline has not changed," he said.

CORRECTION (Jan. 3, 2020, 10:40 a.m.): An earlier version of this story incorrectly included former national security adviser John Bolton on a list of administration officials subpoenaed by House Democrats during the impeachment inquiry. Although some Democrats called for a subpoena for Bolton's testimony, the House did not formally issue one.