(CNN) It had been weeks since the House of Representatives passed articles of impeachment , and back home senators were getting pushed on what they believed a fair impeachment trial would look like.

Republican Sen. Susan Collins wanted answers from Majority Leader Mitch McConnell about the next phase of the impeachment process.

So, on the first day back to Washington after the holiday recess -- Monday, January 6 -- Collins brought a copy of the Senate's 1999 Clinton impeachment resolution to the floor of the chamber, and handed it to McConnell's staff. She wanted more than a promise that McConnell was going to follow the 1999 Clinton impeachment trial model, something he'd said over and over against publicly and behind closed doors. Collins wanted to see it in writing.

What transpired has been weeks of negotiations between the leader's office and aides to Collins, McConnell and Sens. Lisa Murkowski of Alaska and Tennessee's Lamar Alexander, according to people involved. The group painstakingly dissected each word of the Clinton resolution, discussing what -- if anything -- would change, as moderates pushed for language that would require a vote on whether senators want to hear from witnesses after the House managers and the President's defense team made their case.

In between talks with moderates, McConnell and his staff brought in some of the most conservative legal scholars in the conference including Sens. Ted Cruz of Texas and Mike Lee of Utah, into the fold, checking in with them about how to deal with witnesses down the line.

That effort in effect aimed to ensure that the plan being hatched to address moderate concerns wouldn't invariably lead to a crumbling of McConnell's conservative coalition -- or more importantly, lead to a collapse of support from the President.

"The leader understands the importance of listening to everybody," said one Republican senator who asked not to be named to freely discuss McConnell's negotiating style. "He understands that some people have more pressing issues than others, and he can sort through difference between members who would just like it to be some other way, and members who really need it to be some other way, and at the same time making everyone feel like they were listened to."

Unknown future awaits Senate trial

The resolution with the impeachment trial rules has yet to be released, but the expectation is it will likely include the language moderates fought so hard for on witnesses

It will likely cut a provision from the Clinton resolution that allowed for a motion to dismiss, a move that would stop the impeachment trial in its tracks and stop Trump from getting a full trial and acquittal Republicans in leadership have argued is invaluable for the President. McConnell has said he has a majority to pass the resolution, a promising sign for the start of a trial that has come after four months of an investigation, a House impeachment vote and a weeks-long game of chicken between McConnell and House Speaker Nancy Pelosi over how to run the trial.

But the upcoming weeks will test McConnell's -- and to some extent Democratic Leader Chuck Schumer's as well -- ability to hold their respective parties together beyond the early days of the trial.

"I don't envy the majority or minority leaders' tasks," Sen. Ron Johnson, a Republican from Wisconsin, told CNN. "It is not a very easy job. You are corralling independent senators from different states with different constituencies."

Up ahead, GOP senators will have to navigate an impeachment with a President who shares his stream of consciousness and evolving wishes for his defense strategy on Twitter.

Schumer, meanwhile, will have to manage a handful of Democratic senators running for President who will be confined to their seats in the Senate instead of crisscrossing the campaign trail just weeks before the Iowa caucuses. Schumer will also have to keep an eye on Democratic Sens. Doug Jones of Alabama and Joe Manchin of West Virginia who will have to decide if the case their House colleagues are making is compelling enough to remove a sitting President from office just months before a federal election.

Both sides will have to handle witnesses in a trial that has no firm end and in an environment where the slow drip of new information threatens to derail carefully-constructed narratives.

"I've said (to Schumer) 'Are you sure you want John Bolton testifying because we don't know what he is going to say," Sen. Chris Coons, a Democrat from Delaware told CNN. "Just recognize we have to be open to getting to the truth as opposed to just getting a political result. (Schumer's) response was 'we want witnesses.'"

A mission to get senators on board with the Clinton model

For months, aides and members say McConnell has laid the groundwork for an impeachment process that would somehow shield his vulnerable members from charges of partiality while ultimately leading to Trump's acquittal. McConnell quickly latched onto the idea of using "the Clinton model," a reference to the very first resolution in the Clinton impeachment trial that passed the Senate 100 to 0 and set up a process that allowed both the House managers and the President's defense team to make their cases before lawmakers voted on hearing from witnesses.

Republicans say McConnell's job to unite them was also helped by what they say were Democratic missteps: Pelosi's decision to withhold the articles for weeks in an effort to extract concessions from a Republican-controlled Senate and GOP perceptions that Democrats are using impeachment to target vulnerable Republicans.

"It's pretty obvious it is not about the President," Sen. John Barrasso, a Republican from Wyoming, told CNN. "It is about Schumer's approach to become the majority leader of the Senate."

McConnell has largely given his members the floor to speak about their concerns in party lunches, refraining from putting his thumb on the scale in too specific of a direction. But, in a process where historical precedent is thin, and the options are many, Republican senators say McConnell has masterfully led his conference to find a unified position. The starting point? A PowerPoint presentation the leader provided to his caucus back in October that set in motion the party's eventual embrace of the Clinton model.

"What McConnell did was explain it was a process that had precedent, that was done previously. I saw Trent Lott recently, who was the Republican leader at the time, and he explained it exactly the way McConnell explained it to us," one Republican senator told CNN about how McConnell slowly convinced the conference to adopt his view of how a trial should run.

In lunches, members and aides CNN has talked to say McConnell tends to be in listening mode, relying on members themselves to articulate to their colleagues why some provisions in an organizing resolution are essential and why others could be a liability back home. It's a strategy, he's used throughout his tenure in the Senate.

"There is no secret sauce here," Sen. Pat Toomey, a Republican from Pennsylvania said Thursday when asked if McConnell's approach this time was any different than his past leadership style.

Fight over witnesses

The weeks ahead will no doubt put strain on the GOP's unity.

On Thursday, Schumer warned Democrats would force votes on witnesses as soon as the official impeachment trial kicks off on Tuesday, leading to questions about where vulnerable Republicans up for reelection in states like Maine, Colorado, North Carolina or even Iowa will land on questions about witnesses.

Conservatives have also warned that if Democrats push for their witnesses, they may force votes on controversial witnesses like Hunter Biden, Joe Biden and even the whistleblower, once again putting moderates in a difficult position of having to decide between backing the President or voting for witnesses that could be perceived back home as overly partisan.

"That is going to be a challenge, and I don't know how this all plays out. On most days, both sides will eat lunch together before the trial and there will probably be a fair amount of discussion," one Republican senator said. "The bottom line is once 51 senators say we have heard enough, this will end."