A Bay Area Rapid Transit (BART) union worker with SEIU Local 1021 walks a picket line in front of the Lake Merritt station on July 1, 2013 in Oakland, Calif. - 

It’s Labor Day -- just 30 days before Americans are expected to begin signing up for health insurance under the Affordable Care Act. And organized labor has over Obamacare.

Some of the nation’s largest unions, including the AFL-CIO and the Teamsters, believe the new health care law could jeopardize health plans for workers across the country.

From high-wage construction workers to low-wage restaurant workers, 20 million employees get their health insurance through something known as Taft-Hartley plans. That’s where unions and employers get together and pay in for coverage.

Marquette University Law Professor Paul Secunda says Obamacare makes those plans really expensive for companies. “It might make more sense for employers to stop offering the Taft-Hartley plans, instead allow their employees to go on the state healthcare exchanges,” he says.

Secunda says not only would that mean workers end up with less generous coverage, it threatens the basic value of the unions.

“One of the thing that employees look to unions for -- the ability to get all sorts of employee benefits, but maybe most importantly health insurance benefits -- will no longer be something the unions can no longer offer in this environment,” he says.

With so much at stake, union leaders are pressing the Obama administration to protect the plans.

“I’m not sure there is anything the administration can do about it,” says Washington and Lee professor Timothy Jost.

Unions argue that if their workers could receive subsidies to help cover the cost, that would keep the Taft-Hartleys alive.

Jost says something like that requires an act of Congress.

“And right now, at least the House doesn’t seem very inclined to help unions,” he adds.

Ironically, Jost says, unions -- who backed Obamacare from the beginning -- get hurt, while for-profit health plans like Aetna and United will end up making a lot of money thanks to the ACA.

Follow Dan Gorenstein at @dmgorenstein