WASHINGTON — Last fall, before the House impeachment hearings had even begun, North Carolina Sen. Thom Tillis was unambiguous on where he stood on the subject. “Let me be clear,” the Republican lawmaker contended in an October email response to a constituent, “this is nothing more than a political exercise to distract the American people from President Trump’s record of results.”

The views expressed in the email were in line with some public statements Tillis made at the time, including a Sept. 25 tweet calling impeachment a “pathetic attempt” to destroy Trump “with falsehoods.”

Now that Trump has been impeached and faces a trial in the Senate, that response from Tillis to North Carolina resident Linda Sand serves as another example of the Democrats' suspicion that the outcome may already be preordained.

And those sentiments, voiced by Tillis and other GOP senators, are why Democratic leaders are insisting that the Senate agree to hear from witnesses during the upcoming trial.

Senate Majority Leader Mitch McConnell, R-Ky., has said he will proceed with a trial without a commitment in advance to calling witnesses. He cites as precedent President Bill Clinton’s 1999 impeachment, in which a deal on witness testimony was reached weeks into the trial instead of at the beginning.

Both the impeachment trials of Clinton and President Andrew Johnson in 1868 eventually included witness testimony.

Once the trial is underway, a simple majority of senators can vote to call individual witnesses — putting pressure on Republicans like Tillis to either side with hearing from witnesses or possibly appear to protect the president from further damaging information.

Tillis is among the top four most vulnerable Republicans running for re-election in 2020, according to the nonpartisan Cook Political Report.

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At the time of his response to Sand, Tillis was under fire from a primary challenger, conservative businessman Garland Tucker, who had spent $1.5 million on advertisements attacking Tillis on issues like his opposition to Trump’s emergency declaration to build a border wall.

Tillis’ challenger dropped out in December. But “for the other endangered incumbents, if for some reason they do break with their party, the filing deadline hasn't passed so it might spur a primary challenger,” Jessica Taylor, a Senate analyst at the nonpartisan Cook Political Report, said.

More recent comments suggest Tillis remains skeptical, including of the need to hear from witnesses. “I don’t want to spend a lot of time doing what I expected the House to do,” Tillis said last Friday on Fox News. “You’d think if there was any truth to it, they would have spent the time in the House to actually bring it forward” by challenging witnesses refusing to testify through the court system, he said.

“This is just another sham in trying to impeach this president for three years,” Tillis said. But national polls show that Americans by a large majority favor hearing from witnesses.

The peril for vulnerable senators like Tillis is apparent. Traditionally Republican-leaning North Carolina is becoming more moderate — Trump won the state by less than 3 percentage points over Hillary Clinton in 2016. And voters like Sand, a college-educated woman with grown children who’s been a Republican for 33 years, represent the kind of voters Tillis will need to win a general election.

Unenviable position

It’s an unenviable position for many senators such as Arizona’s Martha McSally and Colorado’s Cory Gardner between the demands of their purple state voters and McConnell, the powerful majority leader who has already signaled Trump will never be removed from office. So far, 3 of the 4 of the Republican senators considered most vulnerable have, at a minimum, expressed skepticism about impeachment.

Sen. Susan Collins, R-Maine, says she’s working with a “fairly small” group of Republicans to arrange for witnesses. The question is how small. Democrats would need at least 4 Republicans to join them to reach the 51 votes needed to demand witness testimony.

In December, Gardner told the Denver Post that impeachment has been “a total circus that has only served to divide this country.”

McSally has also been more quiet. Her campaign manager said in a December statement to The Associated Press that she "takes her role as a juror seriously but hasn't heard anything so far that would lead her to believe impeachment of the president is warranted, let alone removing him from office.”

While Clinton, during his impeachment trial, cooperated with congressional requests, Trump has blocked documents and prevented his chief deputies from testifying. A notable break came earlier this month when former national security adviser John Bolton offered to testify in a Senate trial if he is subpoenaed. The House decided not to subpoena Bolton after he refused a request to testify voluntarily.

GOP moderates not facing re-election this year could also hold sway over the calling of witnesses, including Lisa Murkowski of Alaska, Rob Portman of Ohio and Utah’s Sen. Mitt Romney. A number of them and others have criticized an incomplete House process they say didn’t give them enough information, vowing the Senate will do better.

Another source of uncertainty comes in the role that Chief Justice John Roberts will play in the decision on whether to call witnesses.

David Super, a congressional and constitutional law expert at Georgetown University, told NBC News that under Senate rules, Roberts, who will preside over the trial, rules on motions to subpoena witnesses. While Roberts could put all such motions to a vote, even that could be seen as partisan, Super said.

“I can’t imagine that Roberts won’t rule (to allow witnesses) because that would be so extraordinarily partisan and he has worked to make sure the court is not seen as a partisan body,” he said. “Presiding officers subpoena witnesses. It’s not controversial. It happens in virtually every trial.”

That means vulnerable Republicans like Tillis would be in the awkward position of voting to overrule Roberts, a conservative Republican appointed by President George W. Bush, in order to prevent witness testimony.

Sand, in an email to NBC, recalled her initial correspondence with Tillis: “I told him that if I felt that he did not do the right and proper thing that I would use my time and resources to support his opponent and unseat him in the next election.”