Two of the most powerful lobbies in Washington, D.C. — the U.S. Chamber of Commerce and the National Restaurant Association — have joined in a lawsuit against Seattle’s new $15-an-hour minimum wage ordinance.

The USCC and Restaurant Association are bedrocks of opposition to President Obama’s call for a more modest hike from $7.25 to $10.10 in the federal minimum wage.

Along with other groups, they have filed an amicus brief supporting a legal challenge brought by the International Franchise Association.

The suit seeks a preliminary injunction against a provision that, the plaintiffs claim, discriminates against franchise owners in the Emerald City.

“Not only does it impose the largest minimum wage increase in the history of the United States, but does so by targeting plaintiffs for disfavored treatment because of their affiliation with out of state franchisors,” the intervenors said in a statement.

The Seattle law gives small businesses, notably family owned restaurants, up to seven years to implement the $15 an hour wage.

But franchised food outlets get treated as large employers, and given a much shorter three to four years to ramp up to the Seattle minimum.

“By saddling them with a massive increase in labor costs that non-franchise small businesses are not required to bear, the ordinance will make it quite difficult — if not impossible — for franchises to compete,” added the statement.

The ordinance was passed and signed into law in June. It takes effect next April.

The national movement for a higher minimum wage — and the local drive for a $15-an-hour wage — was touched off by wildcat strikes at fast food outlets, demonstrations by fast food workers, and a mass march from Sea-Tac into downtown Seattle.

Seattle Mayor Ed Murray has pointed out that franchisees receive corporate support in terms of advertising, food supplies and menu design.

“They are not the same as a new restaurant that opens up in the city, so I think they are different,” Murray said in a statement responding to the suit.

“I don’t think the strain is on a fairly slow phase in on the minimum wage, but on a business model that really does, in many cases, harm some of the franchise owners.”

Other groups joining in the suit include the American Hotel and Lodging Association, and the Washington Retail Association.

The plaintiffs are represented by Paul Clement, former U.S. Solicitor General under President George W. Bush.

Clement was in the news last year. He was retained by Republican leaders in the U.S. House of Representatives to defend the federal Defense of Marriage Act. He argued before the U.S. Supreme Court that it should uphold DOMA. But the high court, on a 5-4 vote, ruled against DOMA, saying that the federal government could not deny benefits to same-sex couples legally married in the states where they reside.