Opponents of Sunoco Pipeline's Mariner East project won seats on the governing boards of two Chester County townships  Tuesday in elections that had become referendums on the controversial pipeline.

Four Democrats, supported by the political arm of a national environmental group, won races for township supervisor in Uwchlan and West Goshen, which are bisected by the Mariner East system that conveys natural-gas liquids to Sunoco's terminal in Marcus Hook.

The candidates — Mayme Baumann and Bill Miller in Uwchlan, and Mary LaSota and Robin Stuntebeck in West Goshen — vowed to use local zoning regulations to prevent construction of the pipeline they say is too close to occupied structures such as schools and homes.

"I think the people spoke loud and clear and want the township to protect its citizens," said Baumann, who took 60 percent of the vote in her first run at elected office.

Sunoco's construction of the second of three adjacent Mariner East pipes has aroused opposition from residents. The project has been hampered by a series of spills and contamination of private water wells.

Energy Transfer Partners LP, Sunoco's parent company, on Wednesday announced that completion of the Mariner East 2 pipeline will be pushed back from year's end to the second quarter of 2018 because of regulatory delays.

Sunoco began shipping propane on the Mariner East system nearly three years ago in an 80-year-old repurposed pipeline. Construction began this year on the 20-inch-diameter Mariner East 2, which would quintuple the capacity of the system. Sunoco plans to build an additional 16-inch-diameter pipeline next year.

The $2.5 billion project, including upgrades to the Marcus Hook terminal, is aimed at creating an eastern outlet for the production of valuable gas liquids such as propane and ethane from the Marcellus and Utica shale formations. Much of the material will be exported to petrochemical plants in Europe.

The insurgent candidates joined a Democratic uprising across Chester County to overcome a Republican voter-registration advantage in the townships. In Uwchlan, the casualties included Joseph E. Toner III, the incumbent chairman of the supervisors and long-standing member of the Chester County Republican Committee.

The Mariner East project was approved by the Pennsylvania Public Utility Commission in 2014, and Sunoco Pipeline has argued that PUC approval overrides the authority of local officials to block the route. A lawsuit arguing that local municipalities can assert zoning control over the pipeline is currently under review by Commonwealth Court.

In Uwchlan, the newcomers will be two of three members of the Board of Supervisors come January. In West Goshen, the new supervisors will hold two of five seats.

West Goshen has been embroiled in a long-standing dispute with Sunoco over the siting of a pipeline valve station on Boot Road near Route 202. Last month, the PUC  dealt Sunoco a setback in West Goshen, keeping an injunction in place prohibiting the company from building the valve station on a new site the township says was not part of an agreement.

On Energy Transfer Partners' quarterly earnings call Wednesday, company officials told investment analysts its engineers are working to relocate the West Goshen valve station to settle the matter.

"We'll resolve this, and we want to be good corporate citizens with all these townships that we go through," said Thomas E. Long, ETP's chief financial officer. "And we just understand sometimes we just have to work with the regulatory authorities and get these things done. It's a different world out there right now."