The No. 2 House Democrat spoke out against a limited participation proposal, floated publicly by some members of the party, to send just one Democrat to the committee to gather documents and monitor the probe but not fully take part.
"My view is that's the worst of all possible worlds. We're sort of there but you have one person, while you have seven people over here. That's not fair, it's not balanced. And it would give a skewed perception of what's going on," Hoyer said. "My view is one person is -- I think you're either in or you're out, you're not sort of half in or 20 percent in."
Progressive activists recently launched a petition aimed at pushing Democratic leaders to appoint Rep. Alan Grayson (D-FL) to the panel.
Hoyer said that if the ratio won't be even (the panel allows for 7 Republicans and 5 Democrats) Democrats want an equal opportunities to call up witnesses, equal access to all documents submitted by the administration and a "full and fair opportunity" to have their lawmakers and counsel cross-examine witnesses.
"We're still in negotiations [with Speaker John Boehner] on that issue," he said.