Although some labor unions enthusiastically supported the Affordable Care Act when it was being developed and debated, many now are concerned that the law's provisions could increase costs for their health insurance plans and make their workers less competitive, the Wall Street Journal reports.

About 20 million U.S. residents are covered under union-sponsored health plans, which are jointly controlled by unions and employers, according to the Journal.

The labor groups are particularly concerned about ACA provisions that:

- Eliminate caps on medical benefits and prescription drugs, which were used to control costs in many health plans; and

- Allow children to stay on their parents' insurance plans until age 26.