Did vulnerable Republican senators just agree to take the fall for Trump in 2020?
Composite image of Sen. Joni Ernst (R-IA), Sen. Susan Collins (R-ME), Sen. Martha McSally (R-AZ), Sen. Cory Gardner (R-CO), Senate Majority Leader Mitch McConnell (R-KY) and Sen. Thom Tillis (R-NC) via screengrabs

A slate of vulnerable Republican senators voted against witnesses being heard in the impeachment trial when they were given the opportunity Tuesday.

Sen. Chuck Schumer (D-NY) filed an amendment to the rules outlined by Senate Majority Leader Mitch McConnell (R-KY) and when Republicans were given an opportunity to vote in support of allowing witnesses, all 53 voted against it.

Sen. Susan Collins (R-ME), who is among the endangered Republicans up for reelection in 2020, tried to hedge her bets, saying that she was "likely" to support witnesses in the future, but she made no promises. It further poses the question, if Collins is "likely" to support witnesses tomorrow, Thursday, or next week, why isn't she willing to do it today?

Other than Collins, there are already three other Republicans on record opposing the nearly 70 percent of Americans who believe the trial should have evidence and witnesses presented.

As Washington Post political analyst and opinion columnist Greg Sargent pointed out, these members are already on video opposing witnesses.

"So it’s worth noting that three of them are already on video from last fall refusing to answer a very simple question: Is it or is it not appropriate to press a foreign leader to help dig dirt on a domestic political rival?" Sargent asked.

"The three Senators -- Cory Gardner of Colorado, Martha McSally of Arizona, and Joni Ernst of Iowa -- all face tough races, though Gardner and McSally are probably more vulnerable than Ernst."

Last week when a CNN reporter asked McSally whether she supported allowing new evidence, like witnesses John Bolton or Mick Mulvaney, McSally flipped out and called the reporter a "liberal hack" for even posing the question. McSally then proceeded to play the victim, saying she was somehow under attack by the media and people should donate to her campaign.

Gardner has tried to lay low. In his state, he's pretended to support local measures and promote the state itself. In Washington, it's another story. Gardner joined with Republicans to block witnesses. The latest polls show Gardner down by 13 points in Colorado.

The unfortunate understanding about these endangered Republicans is that if they've already shot themselves in the foot, there's little reason for them to fight for their reputation or independence in the impeachment trial. It may be that they've already decided they'll take the bullet for the president and hand their seats over to Democrats, and the Democratic majority in the Senate along with it.

Read Sargent's full analysis.