"Without a good, solid, public option, we will not vote for it," Rep. Lynn Woolsey (D-Calif.) told Washington Post blogger Ezra Klein.
"But this one, we have a line. There are 80 members," she added. "And we have drawn a line in the sand. And we're serious about it."
Woolsey's statements could complicate the effort to get healthcare reform through the House and Senate, which will require the support of centrist Democrats to overcome Republican opposition to the bill.
Woolsey also signaled that the creation of a so-called "co-op" option, proposed this week by Sen. Kent Conrad (D-N.D.) and an idea around which Republicans have rallied, would not be a sufficient compromise for the liberal Democrats.
"I'm not impressed with that idea myself," she said. "I would prefer a plan based on the Medicare model. We know it works! Why try something that we don't know will work?"