Regina Seltzer was 70 years old, recently widowed and a grandmother in 2000 when she pulled off one of the greatest political upsets in Long Island history.

The Bellport attorney, upset that wealthy Republican Rep. Michael Forbes was changing parties to run for re-election to Congress as a Democrat, launched a quixotic primary challenge against him.

“What do you need this for?” her son, Eric Seltzer of Katonah, remembered asking his mother. “Nobody else was willing to do it, so she took the bull by the horns and did it herself.”

She beat Forbes by 35 votes, but lost the general election to Republican Felix Grucci. Seltzer’s effort was emblematic of a career in public life marked by her passionate defense of treasured causes.

Seltzer, remembered by friends and colleagues as a real-life David who locked horns with the Goliaths of local government, took her own life Wednesday at her home, her son said. She was 86.

A Patchogue-Medford school librarian earlier in her career, Seltzer served one term on the Brookhaven Town Board from 1976 to 1979 and began a six-year stint on the town Planning Board before receiving a law degree from Hofstra in 1983, when she was 53.

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Seltzer, known as Reggie, represented the Long Island Pine Barrens Society in more than a dozen lawsuits to protect threatened woods and wetlands.

“When she made up her mind about something, she didn’t let go,” said Richard Amper, the society’s executive director. “She just said, ‘That’s the way it’s going to be.’ ”

Born in Poland, Seltzer and her family fled the country before World War II when Seltzer was 6. They lived briefly in pre-Israel Palestine before coming to the United States and settling in the Bronx.

Her husband, Stanley, a biochemist at Brookhaven National Laboratory in Upton, died in 1999.

An avid gardener, Seltzer played tennis, enjoyed bird-watching and wrote a mystery novel about Benedict Arnold. She was a member of the Audubon Society and the League of Women Voters.

She returned to her legal career after the loss to Grucci and continued to work until her death, most recently representing South Country Ambulance District residents who questioned a proposed ambulance building in North Bellport.

Last year, Seltzer stood and protested at a Brookhaven Town Board meeting when a man who had spoken against the plan was escorted out by town security for speaking beyond a time limit.

Regina Seltzer, a Democratic candidate for the 1st Congressional District, in her Bellport home with petitions to run for the office that Michael Forbes now occupies. Photo Credit: Daniel Goodrich

The diminutive giant slayer — she stood less than 5-feet tall — challenged government officials with chutzpah and solid legal research, earning admiration even from some opponents.

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“Even when you disagreed with her, she was always personable,” Brookhaven Supervisor Edward P. Romaine said. “She was always pleasant, but she made her point.”

Former Suffolk County chief deputy executive Paul Sabatino said he had been skeptical when he first encountered Seltzer, whose style he described as “the sledgehammer approach to law.” But he came to admire her as she piled up legal victories.

“She would basically take everything she had in terms of legal arguments, and she put it on the edge of the sledgehammer and banged it in,” Sabatino said. “I stand corrected: She was right.”

In addition to her son, Seltzer is survived by Eric Seltzer’s wife, Nealle, and their daughters, Veronica Jean and Bryn. Seltzer’s other son, Ronald, predeceased her.

Eric Seltzer said no services are planned, though a memorial service may be held at a later date. Seltzer’s body will be cremated, he said.