"Bottom line, I was very upset when I heard Sen. Reid say that about Romney because it's based on one conversation and it could really smear somebody," Lieberman said. "I know that obviously that Democrats are now saying Romney could end this by now putting out his tax records, but he's said he doesn't want to put out his tax records for 10 years."
The Connecticut lawmaker also said that he thought the tactic violated his childhood code of ethics.
"One of the things I grew up with was the idea that if you're talking about someone else, you really should not be speaking unless you know for sure, and frankly even if you know something bad about him, you shouldn't say that about him either," Lieberman said.
Over the weekend, House Minority Leader Nancy Pelosi (D-Calif.) defended Reid's reporting of his conversation with the Bain investor.
"Harry Reid made a statement that is true. Somebody told him. It is a fact," Pelosi said to The Huffington Post on Sunday. "Whether he did or not can easily be disposed of: Mitt Romney can release his tax returns and show whether he paid taxes."
But Republicans have blasted Reid's remarks, with Sen. Lindsay Graham (R-S.C.) saying he believed Reid was "lying."
Romney on Friday said Reid needed to "put up or shut up" about the report.
“Harry's going to have to describe who it is he spoke with because of course, that's totally and completely wrong," Romney said while campaigning in Nevada. "It's untrue, dishonest and inaccurate. It's wrong. So I'm looking forward to have Harry reveal his sources and we'll probably find out it's the White House."