“Impeach Trump — the evidence is already there,” a caller named Pam said. “It’s just a matter of exposing it to the public. We won’t change Republicans, Cliff. Maybe independents, perhaps.”

WDSM 710 AM Duluth, Minn.: ‘How do we know there is a whistle-blower?’

As far as Steve from Duluth is concerned, the impeachment public hearings should not proceed until the identity of the anonymous whistle-blower who started the whole inquiry is revealed.

If there actually is a whistle-blower, he said.

“My statement would be, you won’t tell us who this whistle-blower is, so this so-called trial is over because how do we know there is a whistle-blower, just cause you say there is a whistle-blower?” Steve asked in a call to “Sound Off,” a weekday radio program in Minnesota. “How do we know there is one?”

Brad Bennett, the host of the three-hour morning show, quickly agreed. “Actually Steve, that is not a bad idea.” He noted that Senator Lindsey Graham, Republican of South Carolina, has said that a Senate trial needed to expose the whistle-blower’s identity to investigate whether there is any relationship between the whistle-blower and Joseph R. Biden Jr., the former vice president and current presidential candidate.

“I thought it was an interesting theory,” Mr. Bennett said, adding, “For example, if they find out who the whistle-blower is and they find out this guy was an operative, worked on the Biden campaign, was involved with him, maybe did some work with him, that would throw the whole thing out. Wouldn’t it?”

On the show, Steve — one in a parade of callers who made it clear they believed that President Trump has been unfairly accused — insisted that the priority should be determining the existence of a whistle-blower at all. “Yeah,” Steve told the host, “but I still think a statement should be put out there that questions, How do we know there is a whistle-blower if you won’t tell us who this person is?”

Others called after Steve.

Tom said he does not view a quid pro quo “as anything other than the president having the legitimate ability to maneuver on foreign policy. I am more concerned not with the government being for sale, but the president being handcuffed.”