Groton ― State Sen. Heather Somers, R-Groton, won a fifth term representing the 18th Senate District on Tuesday.

Somers defeated Democrat Andrew Parrella 27,950-18,264 in the eight towns in the district.

Somers spent Election Night at the home of Eva Franchi in Stonington as Republicans kept track of votes in the sprawling district, which encompasses Groton, Stonington, North Stonington, Griswold, Sterling, Voluntown, Plainfield, and Preston.

“I feel cautiously optimistic,” Somers said in a brief interview. “This is my passion, and I try to serve people no matter what party you’re in.”

Parrella lost every district in Stonington except for the Borough, where he and Somers were in a tie at 479 votes each. Somers won all the voting districts in Groton, where both she and Parrella live. Parrella lost to Somers in Sterling by nearly 1,000 votes, and in Plainfield by more than 400.

Parrella, contacted about 9 p.m. Tuesday, was not ready to concede with absentee and early ballots still to count, but he acknowledged the odds were against him.

“She’s a very, very formidable opponent,” he said. “Senator Somers has done a lot of things that have indicated she’s willing to cross lines ... and that has helped her.”

Of all the towns in the district, Groton is by far the largest (over 23,000 registered voters), followed by Stonington (15,458) and Plainfield (10,243). In all, the district had 66,359 registered voters, according to the secretary of the state’s office.

“You have to do well in Mystic and Stonington or you have to do well up north,” Somers said in a phone interview assessing her path to victory before any results were in.

Somers, a one-time lieutenant governor candidate who made a name for herself by challenging the administration of the Whiting Forensic Institute and what she saw as a too-soft approach to parole for convicted criminals, ran her campaign based largely on her government experience, which included a stint as Groton town mayor.

Parrella is a former Groton city councilor and a member of the Representative Town Meeting who works as an executive host at the Mohegan Sun. Parrella ran on his record of more than 30 years in local government, including a brief stint as a Republican, and said his big issues were affordable housing, the environment, education, health care and labor rights.

l.howard@theday.com