MILWAUKEE — A conservative group filed a federal lawsuit Wednesday seeking to have the federal health care law declared unconstitutional, arguing that the government postponed its so-called employer mandate without the necessary congressional approval.

The Association of American Physicians and Surgeons is seizing on a provision that was supposed to go into effect next year but ultimately was postponed for 12 months. That provision requires employers with 50 or more workers to offer affordable coverage or face fines, but it’s been put off until 2015.

That employer mandate was originally to go into effect at the same time as a mandate requiring individuals to carry health insurance or pay fines. The lawsuit argues that since the two aspects have been separated, people will now have to spend more on premiums because they’ll be required to buy insurance without the “protection” of the employer mandate.

The Association of American Physicians and Surgeons is a politically active group that favors having patients pay directly for medical services without the intervention of health insurance companies. It sued on behalf of Robert T. McQueeney, a psychiatrist from northeastern Wisconsin who doesn’t accept most forms of insurance and whose clients often pay in cash.