Labor unions that have solidly backed President Obama are splitting with him over ObamaCare -- with one calling for the “repeal or complete reform” of the president’s signature health-care law.

Union leaders argue insurance costs for millions of workers will increase under the president’s health-care plan so they might have to drop their existing plan, despite Obama promising the opposite.

Their primary concern is the multi-employer or so-called Taft-Hartley plans that cover unionized workers in retail, construction, transportation and other industries that frequently use seasonal and temporary employment.

The union leaders say the roughly 20 million people covered by the plans will likely have higher premiums because the Affordable Care Act does not include tax subsidies for them.

However, workers seeking coverage in the upcoming, state-based marketplaces for insurance, known as exchanges, can qualify for subsidies.

Union leaders are now hearkening back to what Obama repeatedly said starting in 2009: "If you like your health care plan, you can keep your health care plan."

Joe Hansen, president of the United Food and Commercial Workers International Union, wrote in a recent op-ed that that scenario “is not going to be true for millions of workers now" and the realization “makes an untruth out of what the president said.”

The plans are jointly administered by unions and smaller employers that pool resources to offer continuous coverage, even during periods of unemployment.

The union plans were already more costly to run than traditional single-employer health plans. And the Affordable Care Act only added to the cost by mandating essentially all plans cover dependents up to age 26, eliminate annual or lifetime coverage limits and extend coverage to people with pre-existing conditions.

"We're concerned that employers will be increasingly tempted to drop coverage through our plans and let our members fend for themselves on the health exchanges," said David Treanor, director of health care initiatives at the Operating Engineers union.

Other unions expressing their concerns include the hotel workers union UNITE HERE, the International Brotherhood of Teamsters and the United Food and Commercial Workers International Union, according to The Hill newspaper.

They are joined in such concerns by at least two congressional Democrats, House Minority Whip Rep. Steny Hoyer, Maryland, and retiring Montana Sen. Max Baucus.

Baucus, the chairman of the Senate Finance Committee, recently said implementing the law could be a “train wreck.”

The bulk of the law is scheduled to go into effect on Jan. 1, 2014.

Bob Laszewski, a health care industry consultant, said the real fear among unions is that many labor contracts are already very expensive and now employers are going to have an alternative to very expensive labor health benefits.

"If the workers can get benefits that are as good through ObamaCare in the exchanges, then why do you need the union?" Laszewski said. "In my mind, what the unions are fearing is that workers for the first time can get very good health benefits for a subsidized cost someplace other than the employer."

However, Laszewski said it was unlikely employers would drop the union plans immediately because they are subject to ongoing collective bargaining agreements.

Labor unions have been among the president's closest allies, spending millions of dollars to help him win re-election and help Democrats keep their majority in the Senate. The wrangling over health care comes as the 2014 elections near and union membership steadily declines amid attacks on public employee unions in state legislatures in Wisconsin and elsewhere across the country.

Union officials have been working with the administration for more than a year to try to get a regulatory fix that would allow low-income workers in their plans to receive subsidies. But after months of negotiations, labor leaders say they have been told it won't happen.

A Treasury Department spokeswoman declined to discuss the specifics of negotiations but said the law helps bring down costs and improve quality of care.

In addition, union officials also reportedly met privately this month with Senate Democratic leaders to discuss the issue.

Unions say their health care plans in many cases offer better coverage with broader doctors' networks and lower premiums than what would be available in the exchanges, particularly when it comes to part-time workers.

Unions backed the health care legislation because they expected it to curb inflation in health coverage, reduce the number of uninsured Americans and level the playing field for companies that were already providing quality benefits. While unions knew there were lingering issues after the law passed, they believed those could be fixed through rulemaking.

"In the rush to achieve its passage, many of the act's provisions were not fully conceived, resulting in unintended consequences that are inconsistent with the promise that those who were satisfied with their employer-sponsored coverage could keep it," Kinsey Robinson, president of the United Union of Roofers, Waterproofers and Allied Workers, said last month. “I am therefore calling for repeal or complete reform of the Affordable Care Act.”

The Associated Press contributed to this story.