Senate Majority Leader Mitch McConnell is leading the charge against Democrats in the Senate trial. He is seen as the architect of the trial and will determine how long proceedings will go on for.
He has previously said that everything he does is in close coordination with the White house.
Sen. Murkowski is considered a key Senate moderate. She has voted against Republican-backed measures, including opposing Brett Kavanaugh's nomination to the Supreme Court in 2018.
She is seen as a dark horse and has said she was "disturbed" by the GOP strategy of total coordination with the White House.
Sen. Mitt Romney is one of the president's fiercest critics, and previously called Trump's request for Ukraine to investigate former Vice President Joe Biden "troubling in the extreme."
In December, he told CNN that it was "too early" to decide whether to remove Trump from office, signalling that his vote could be up for grabs.
Sen. Collins has said she is open to calling witnesses as part of the trial, but said it would be "premature" to decide who those witnesses may be until presented with further evidence.
"It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us,″ Collins told Maine Public Radio in December.
In a statement on Tuesday, Collins said that while she voted to table attempts to subpoena documents or witnesses before hearing the case, she said that it would be likely that she would support a motion to subpoena witnesses after additional questions are answered.
Sen. Graham is the chairman of the Senate Judiciary Committee and a staunch Trump ally. He also has past experience with impeachment proceedings and acted as a House manager in the impeachment trial of former President Bill Clinton.
He has denied that he is participating in a "cover-up" to protect the president.
The outspoken senator has also suggested that Democrats are on a mission to "destroy" Trump at all costs.
I spent 2 weeks in India. A highlight was visiting a small mountain town so beautiful it didn't seem real. 
I quit McKinsey after 1.5 years. I was making over $200k but my mental health was shattered.
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