(CNN) Former national security adviser John Bolton's surprise announcement Monday that he is willing to testify in the Senate's impeachment trial significantly raises the already massive stakes of the pending votes in the chamber as to how the trial of President Donald Trump will be conducted. And it puts even more pressure on Senate Majority Leader Mitch McConnell as he seeks to keep his 52 GOP colleagues in line as both sides prepare for the historic proceeding.

At the core of the debate over how the Senate impeachment trial will work is whether or not witnesses will be allowed to be called. In mid-December, Senate Minority Leader Chuck Schumer sent a letter to McConnell requesting that four witnesses be allowed to testify -- a list that included Bolton as well as acting White House chief of staff Mick Mulvaney.

McConnell promptly rejected that proposal from Schumer. "We don't create impeachments," McConnell said on the Senate floor. "We judge them."

But here's the catch: The rules of how a Senate trial will work -- with regard to calling witnesses and all the rest -- are determined by the Senate. In the last impeachment trial -- for Bill Clinton in January 1999 -- the rules package governing how the trial would proceed was approved 100-0. (Three witnesses were allowed to testify in that trial.) While McConnell has signaled that he will try to negotiate a deal with Schumer over the rules, it's hard to see how the two will find common ground -- especially over the question of witnesses.

All of which means that there will be a battle royale between the two leaders -- as Schumer seeks to lure four Republican senators to support his push for witnesses while McConnell tries to keep a majority in support of holding the line.

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