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(CNN) As the impeachment trial of President Donald Trump begins on Tuesday, many questions remain about how the process will play out and how it will ultimately impact Trump. This much is for sure: the stakes are stratospheric — for Trump, for those who surround him and for the United States.
Here's what to watch for during the opening week of argument at the trial:
What are the stakes? On the most basic level, Trump's presidency is on the line. In the extremely unlikely event that two-thirds of the Senate — 67 of 100 senators — vote to convict, Trump will be immediately removed from office and Vice President Mike Pence will be sworn in as the new president.
Even if he is not convicted and removed from office, Trump's political outlook for the 2020 election, and his ultimate place in history, will turn largely on the strength of the evidence and arguments brought by the team of House managers who will make the case for impeachment.
Beyond Trump, the trial carries broad implications for people in his orbit who participated in the plan to withhold foreign aid from Ukraine: Rudy Giuliani, Mick Mulvaney, Mike Pompeo and others. And the political futures of the senators who sit as jurors in judgment of Trump could turn on how they vote — not only on the critical guilty-or-not-guilty question but also on key process questions. Most importantly ...
Will we have witnesses at trial? This key procedural issue remains unresolved as we head into trial. Senate Majority Leader Mitch McConnell has steadfastly refused to accede to demands by Senate Minority Leader Chuck Schumer and others to call witnesses at the trial. McConnell has endorsed the "Clinton model" — based on the procedure used in the 1999 impeachment trial of President Bill Clinton — which would put off the question of witnesses until after the parties make opening arguments.
There are three possible ways this could be resolved. First, the parties could come to a negotiated agreement. While McConnell and Schumer appear to be at an impasse, neither have shut the door on the possibility of additional talks.
Second, either party could bring the issue to Supreme Court Chief Justice John Roberts, who sits as the trial's presiding officer pursuant to the Constitution, for a ruling. The Senate's own internal rules permit Roberts to rule on evidentiary issues, which would include witnesses.
Third, the Senate itself could decide. Under the rules, Roberts can either rule on evidentiary issues or put the question to a majority vote of the Senate. The Senate rules provide that if Roberts does rule, the Senate can then overrule him by majority vote (though the Senate rule giving itself the ability to overrule Roberts is in tension with the Constitution's explicit and unqualified command that the chief justice "shall preside").
Will any senators break ranks and vote across party lines? At this early juncture, there is no strong indication that any of the Senate's 53 Republicans will vote to convict Trump or that any of the 47 Democrats (including two Independents who caucus with Democrats) will vote to acquit.
But there are signs that several Republicans could vote with Democrats in favor of calling witnesses. Republican senators including Susan Collins, Mitt Romney, Lisa Murkowski and Lamar Alexander have expressed some willingness to vote for witnesses. If four Republicans make the leap across the aisle and vote with Democrats, then witnesses will be in play, which would ratchet up the level of drama and unpredictability at trial.
Will we see new revelations? Since the House impeached Trump on December 18, 2019, we have seen a steady flow of new information relating to Trump's scheme to withhold foreign aid from Ukraine. A previously undisclosed batch of emails showed that an Office of Management and Budget official ordered the aid withheld on "clear direction from POTUS." Former national security adviser John Bolton announced that he would testify if subpoenaed by the Senate. And Lev Parnas, the indicted associate of Giuliani, publicly detailed his work with Giuliani and others to execute the Ukraine scheme. (Trump continues to deny any wrongdoing, referring to his Ukraine call as "perfect.")
It seems like a safe bet that the drumbeat of new revelations will continue during the impeachment trial. Largely because key witnesses like Mulvaney and Pompeo have followed White House instructions and refused to testify thus far, much remains unknown. Given that void, journalists will continue to uncover new facts, and perhaps more people will be willing to come forward and speak. There is no reason to expect the outside world to remain silent as the trial happens inside the well of the Senate.
Much remains unknown as the trial gets underway in earnest this week. The world, and history, will be watching.