One of the country’s most prominent union officials is warning that big labor may pull its support from Democrats who don’t fight for a government-run insurance plan.

In an interview with the Huffington Post on Saturday, Richard Trumka, the secretary-treasurer and likely next president of the AFL-CIO, said his federation is drawing a line in the sand when it comes to a public option in the health care bill. Lawmakers who don’t support the provision, he said, shouldn’t take anything for granted.

“We’ll look at every one of their votes,” Trumka said after his speech at the Netroots Nation convention. “If they’re against the Employee Free Choice Act, if they’re against health care for that reason, I think it’ll be tough for them to get support from working people.”

Trumka’s remarks were echoed privately by several other labor officials at the convention in Pittsburgh. In particular, the emerging Senate Finance Committee plan – which seems unlikely to contain a public option and could end up taxing pricey health care packages – seems almost guaranteed to incite the unions.

“We’ll oppose it,” Trumka said, when asked about any bill that ends the tax exemption for employer coverage. “It’s actually a stupid concept because if you tax those that have it to pay for those that don’t, eventually those that have [benefits] won’t. Then who do you ultimately tax?”