Washington (CNN) The Senate impeachment trial is starting to get real . After weeks of senators seemingly ducking or dodging the Mack truck headed the chamber's way, Senate Democratic Leader Chuck Schumer kicked the preparations for next year's trial into gear Sunday night by laying out Democratic demands for how that trial should actually look. Let the negotiations begin.

Bottom line

This is an important marker -- and one that diverges sharply from the trial preferences of Senate Majority Leader Mitch McConnell. It shows two things: Schumer isn't going to buckle to McConnell's desire to have a witness-less trial, and that the top Democrat is actively looking to bring moderate Republicans into the Democratic fold on his strategy. The former is crucial for Schumer's Democratic constituency, the latter could dramatically reshape everyone's expectations for what this trial would entail.

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CNN's Lauren Fox has a really great rundown of exactly what Schumer is asking for. Here's the actual Schumer letter

What to watch this week

Schumer put his cards on the table. He and McConnell still plan to meet to see if a bipartisan agreement on how the trial should be structured is possible. That could happen as soon as this week, but nothing is finalized yet.

McConnell vs. Schumer

There are elements of Schumer's three-page letter McConnell would likely sign onto, primarily the structure of the House manager and White House defense presentations, both of which line up with President Bill Clinton's 1999 trial. Even the timing roughly lines up to what McConnell has been hinting at publicly.

But McConnell has made clear privately he's opposed to having witnesses during the trial, preferring instead to have the manager and White House presentations, then move to a final vote on the articles themselves.

Schumer has now made it clear that's a non-starter for Democrats. Consider it a very important marker, one that will dictate Democratic positioning for the next month, laid down.

Schumer's witness requests:

Mick Mulvaney, acting White House chief of staff

John Bolton, former national security adviser

Robert Blair, senior adviser to the acting White House chief of staff

Michael Duffey, associate director for national security, Office of Management of Budget

The White House on Monday will review Schumer's proposal for the Senate trial, an official involved in the matter told CNN's Kaitlan Collins. Officials discussed the broad outlines of it Sunday, but are expected to do so more fully in person on Monday.

As for why Schumer isn't suggesting that former Vice President Joe Biden's son Hunter Biden testify, the New York Democrat told CNN's John Berman on "New Day" Monday that calling him would be "a distraction."

Republicans "have not shown a single bit of evidence that Hunter Biden can answer any of these facts. He's a distraction," Schumer said, adding, "This shouldn't be for Democrats to bring in their favorite conspiracy theories and Republicans to bring in theirs. This is an august and solemn proceeding."

Notable

Schumer makes clear Democrats want the above witnesses because they have "direct knowledge of Administration decisions regarding the delay in security assistance funds to the government of Ukraine and the requests for certain investigations announced by the government of Ukraine."

The use of "direct knowledge" is intentional here as a marker for witness negotiations.

The only thing that matters

The number 51. With 51 votes, McConnell can do most anything he wants in the looming Senate impeachment trial. But at this stage, McConnell doesn't have the commitment of at least 51 of the 53 senators in his conference to do anything. That means what happens next -- and the extent of Schumer's actual leverage right now -- are still unknowns.

The Schumer calculation

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It's not complicated. Schumer, and his fellow Democrats, need to find four Republican senators who back some, or all, of their trial strategy and structure. As one Democratic senator told me last week: "That's the ballgame. We need four."

Schumer's three-page letter to McConnell was setting down a marker with the Kentucky Republican, yes, but it was also sending a message to Republicans who haven't gone all in for President Donald Trump's acquittal yet.

Republican rebuttal

The Republican argument pushing back against Schumer's proposal for four witnesses is starting to take shape. Senior GOP aides are making the case that it's not the job of the Senate to do the fact-finding that the House was supposed to do. The argument is expected to go like this: If House Democrats really wanted to speak with Mulvaney, Bolton and others, then they should have gone to court to secure their testimony.

Sen. John Cornyn tweeted a similar argument, saying that Schumer "should have talked" to House Speaker Nancy Pelosi and Reps. Adam Schiff and Jerry Nadler about his request for witnesses.

A smart way to view this

Everything Schumer does from here on out will be designed to attract GOP support to his effort. Every statement, every position, every public utterance. Without Republican support, he has no power. With it, he can dictate the terms of this trial.

The GOP senators

There's no official list, but in conversations with Democratic senators and aides over the last week, here's a rough list of Republicans they are looking to at the moment. Remember, they need four from this group to be able to start moving anything in their direction.

Of note: Nobody in this group has said they'd be willing to join Democrats on their push for witnesses or a specific trial structure yet. But they also haven't rejected the possibility. It's a mix of moderates, those retiring at the end of this term and, well, Mitt Romney.

Sen. Susan Collins of Maine

Sen. Cory Gardner of Colorado

Sen. Lamar Alexander of Tennessee

Sen. Mike Enzi of Wyoming

Sen. Lisa Murkowski of Alaska

Sen. Mitt Romney of Utah

Sen. Pat Roberts of Kansas

Schumer's bet

Schumer's bet is that at least four of the above Republicans want -- or need -- more than just two presentations to deem the impeachment trial fulsome and legitimate. Whether it's because they are in tight reelection races, or believe it's important to the legitimacy of the trial, or even the legitimacy of a fractured Senate.

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The fact that Schumer, who has held his cards on what he wants to see in a trial extremely close the vest, decided to send his letter and start his public blitz at the same time McConnell and others were touting close coordination with the White House and the merits of ending the trial quickly is not a coincidence. The bet is that the position of McConnell and Sen. Lindsey Graham makes some Republicans more amenable to Schumer's efforts.

Important technical difference

The above represents the political and strategic calculations and strategies. Here's a technical difference between McConnell and what Schumer wants that is important: Schumer, according to his letter, wants the trial structure and witnesses all to be included in a single resolution. That would be different from the Clinton trial. That is very different from McConnell's stated preference.

What it all means for the McConnell-Schumer negotiations

There are sharp differences between what Schumer wants and what McConnell has signaled, either publicly or privately, he would like to see for a trial. That obviously doesn't bode well for the looming negotiations between the two. But it also doesn't short-circuit them outright -- there are legitimate reasons both would want to reach an agreement in one form or another.

The two will meet and try to hammer something out, but keep in mind, the Senate does have rules for an impeachment trial on the books. If no agreement is reached on the bigger issues, the trial will still move forward. But it will be dictated entirely by whatever can get the support of 51 senators.