Leaders of four Democratic caucuses representing more than 120 members of Congress said Wednesday that they would vote against any health overhaul legislation that excludes a “robust” government-run insurance plan to compete with private insurers.

The leaders of the black, Hispanic, Asian Pacific American and Progressive caucuses said at a news conference that they would consider a government-run plan to be robust if it resembles Medicare, the health entitlement for the elderly. The plan would have to be available to everyone in the country and could not be subject to a “trigger,” or some other mechanism that might delay its implementation, the lawmakers said.

Rep. Lynn Woolsey of California, co-chairwoman of the Progressive Caucus, said the groups’ statement was unusual. Typically, leaders of the caucuses do not publicly challenge their party leadership, preferring to work behind the scenes to win concessions in legislation, she said.

“What we’re telling you this time: it’s different,” she said. “Not that we’re going to vote with Republicans. But if reform legislation comes to the floor and doesn’t include a real and robust public option, we will fight it with everything we have.”

The four caucuses represent some of the most liberal members of Congress; in the House, many of their members have signed onto bills that would create a nationalized, single-payer health system by expanding Medicare to cover everyone. Those lawmakers consider the creation of a government-run plan to compete with private insurers a compromise.

Their warning could mean trouble for Democratic leaders, especially in the Senate, where Finance Chairman Max Baucus , D-Mont., is trying to win Republican support for an overhaul he is drafting. Republicans roundly oppose creating a government-run insurance plan, and Baucus has said he is willing to compromise on the topic, perhaps by creating a system of publicly owned insurance “cooperatives” instead.