This week’s Democratic presidential debate in Iowa was the last chance for the significantly winnowed field to make a big impression — not just before the Feb. 3 caucuses there but also before the Senate impeachment trial of President Donald Trump begins.

Of the six Democrats onstage Tuesday night at Drake University in Des Moines, three of them — Sens. Elizabeth Warren, Bernie Sanders and Amy Klobuchar — will be jurors in the upcoming trial and not free to move about the country, as the old Southwest Airlines ads went.

A fourth, former Vice President Joe Biden, could be called as a witness. A fourth senator running who was not on the debate stage, Michael Bennet, will also be a juror.

This is one of the nightmare scenarios for candidates in a tight race looking to pull ahead at an opportune time.