A Cape Cod town and other local opponents of the Cape Wind energy project are seeking another chance to block the long-planned $2.6 billion offshore facility.

The town of Barnstable, the Alliance to Protect Nantucket Sound and local businesses and residents filed a legal brief with the U.S. Court of Appeals in Boston this week seeking to overturn a lower court decision dismissing their lawsuit, which alleged that the state illegally coerced utility NSTAR to purchase power from the Cape Wind project.

The plaintiffs argue that U.S. District Court Judge Richard Stearns' ruling in May was "gravely flawed" because it concluded that their lawsuit was barred by the U.S. Constitution's 11th Amendment, which largely shields states from lawsuits in federal court made without their consent.

Laurence Tribe, a Harvard law professor who is among a team of lawyers who authored the appeal, says Stearns' ruling would make the constitutional amendment a "veritable engine of destruction" for otherwise valid constitutional challenges to state laws, policies, and actions.

Cape Wind seeks to build the first commercial-scale offshore wind facility in the U.S., proposing more than 100 turbines in Nantucket Sound. The project is projected to generate enough power to supply most of the electricity needs for Cape Cod, Nantucket and Martha's Vineyard.

Alliance to Protect Nantucket Sound President Audra Parker said Wednesday that opponents still believe they have a strong case, despite the May dismissal. "This brief paints a troubling picture of the state's actions and its ongoing efforts favoring Cape Wind, which harm consumers and close the door on other, more viable green energy options," she said in a statement.

Cape Wind spokesman Mark Rodgers pushed back at that assertion: "Project opponents are making the same failed arguments they made last time, which the judge called 'misleading and ultimately untrue.'"

Krista Selmi, a spokeswoman in Gov. Deval Patrick's Executive Office of Energy and Environmental Affairs, said the administration is reviewing the appeal.

In their original lawsuit, filed in January, opponents alleged state regulators discriminated against out-of-state power companies by brokering a deal for the Massachusetts-based NSTAR to buy power from Cape Wind.

They also said state regulators exceeded their authority in setting wholesale rates for the contract, in violation of both the Federal Power Act and the Supremacy Clause of the U.S. Constitution, which reserves that power for the Federal Energy Regulatory Commission.

NSTAR has agreed to purchase power from Cape Wind at a starting price of 18.7 cents per kilowatt hour. That, opponents have argued, is well above typical wholesale prices and will lead to higher electricity costs for local ratepayers. The appeal, as with the original lawsuit, names the state, Cape Wind and NSTAR as defendants.