An excellent question!

Thomas Massie questioned whether Democrat senators running for president should recuse themselves from a possible impeachment trial of Trump.

Source: Massie: Should Senators Running for President Recuse Themselves?
Rep. Thomas Massie (R-KY) questioned Wednesday whether Democrat senators running for president should recuse themselves from a possible impeachment trial of President Donald Trump, suggesting it serves as a possible conflict of interest.

Rep. Massie, a member of the House Oversight and Reform Committee, wrote Wednesday whether 2020 Democrat presidential candidates in the Senate have a “conflict of interest” when it comes to a potential impeachment of President Trump.

He added, “Wouldn’t anyone so positioned against a defendant be barred from serving as a judge or jury in a court of law?”

Again, excellent question!!!