ST. PAUL — A contentious fight over union organizing among certain home-based child care and home health care workers in Minnesota is headed back to court.

Opponents filed a lawsuit in federal court Wednesday challenging a law signed just days ago. It gives the American Federation of State, County and Municipal Employees and the Service Employees International Union a window to press for a unionization vote.

A group of child care providers, some of whom successfully sued to block a unionization effort last year, is seeking an injunction blocking a union election.

Their attorney, Doug Seaton, argued throughout legislative hearings on the bill that federal labor law pre-empts state law in this area. Seaton makes the same argument in the lawsuit — that family child care providers are subject to the National Labor Relation Act, which prohibits “employers and independent contractors” from voting in union elections.

“By providing for an election of family child care providers, who are properly defined as employers,” the lawsuit argues, the new law “is in direct conflict with the National Labor Relations Act and the Taft-Hartley Act.”

Child care providers Hollee Saville of St. Michael and Becky Swanson of Lakeville, who have long been active in fighting the unionization effort, are two of the plaintiffs.

“If the unions have their way, we will all look like carbon copies,” Swanson said.

The lawsuit only applies to child care providers, but the law also allows some personal care attendants to vote on unionization.

Jennifer Munt, spokeswoman for AFSCME Council 5, which is organizing the child care workers, said the law merely gives providers the right to vote on unionization.

“There is nothing more constitutional than the democratic right to vote,” Munt said.

The Democratic-controlled Legislature approved the organizing after an earlier attempt by Gov. Mark Dayton to allow it through executive order was overturned in court.

Advocates argue the unions could secure higher reimbursement rates and better industry standards. Opponents have complained that a union would compel independent business owners to join and pay dues regardless of their stance.

Among the named defendants are Dayton, the Bureau of Mediation Services and the Department of Human Services.

In a statement, Rep. Michael Nelson, DFL-Brooklyn Park, the bill’s House sponsor, called the lawsuit “frivolous” and said that last year’s lawsuit paved the way for this year’s law.

“Last year’s court ruling made it crystal clear that the legislature had the authority to pass legislation allowing child care workers to have the opportunity to vote on whether or not they wish to form a union,” his statement read. “No one is forcing anyone to join a union.”