TALLAHASSEE — After acknowledging this month that the initiative might not make the 2016 ballot, backers of a proposed solar-energy constitutional amendment have filed a federal lawsuit against a petition-gathering firm, court documents show.The group Floridians for Solar Choice filed the lawsuit last week in South Florida amid a contract dispute with the petition firm PCI Consultants Inc. The dispute, in part, has included PCI holding 212,000 signed petition signatures as it seeks payment for expenses that Floridians for Solar Choice contends are beyond what the group agreed to pay, according to the lawsuit.The case also appears to make clear that Floridians for Solar Choice will not meet a Feb. 1 deadline for submitting 683,149 petition signatures to get on the November 2016 ballot. The group had submitted 273,280 signatures to the state as of Tuesday afternoon. The process also involves getting petitions validated by county elections supervisors.One line in the lawsuit, alluding to PCI holding petitions, said the "failure to deliver the signed petitions to the counties rendered it impossible for FSC (Floridians for Solar Choice) to have its proposed amendment placed on the ballot."Along with filing the lawsuit last Wednesday, Floridians for Solar Choice requested that the case be sent to arbitration. As of Tuesday afternoon, PCI had not filed documents in response to the lawsuit, according to an online docket.The Floridians for Solar Choice initiative, which has been heavily supported by the Southern Alliance for Clean Energy, seeks to increase the use of solar power by taking steps such as easing regulations. In part, it would allow businesses to generate and sell up to two megawatts of solar power to customers on the same or neighboring properties.But the proposal has faced fierce opposition from major electric utilities and spawned a rival solar-energy ballot initiative by a group known as Consumers for Smart Solar. That utility-backed initiative, which had raised and spent about $5.9 million as of the end of November, had submitted 519,583 petition signatures as of Tuesday afternoon.Floridians for Solar Choice, by comparison, had raised about $1.5 million as of the end of November. Of that total, it paid about $1.2 million to PCI for petition-gathering, with payments dated from Feb. 6 to Nov. 19, according to a state campaign-finance database. In the lawsuit, Floridians for Solar Choice said it terminated the contract Nov. 19 because of the dispute about expenses.In an interview Dec. 18 with the News Service of Florida, Stephen Smith, executive director of the Southern Alliance for Clean Energy, said initiative supporters had started exploring options for appearing on the 2018 ballot, calling the decision a "strategic pivot point."