CHICAGO (WBBM/CBS) — Lesbian activist Kelly Cassidy will replace state Rep. Harry Osterman in the Illinois House, as Osterman moves to the Chicago City Council.

Cassidy, a longtime assistant in the Cook County State’s Attorney’s office, was chosen Saturday from a field of 23 candidates.

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The 14th state House District where Cassidy is serving includes the Edgewater and Andersonville neighborhoods, and most of the Rogers Park neighborhood. It is bounded roughly by Fargo, Birchwood and Rogers avenues on the north, the CTA Red Line and Lake Michigan on the east, Foster Avenue on the south, and Ridge and Ashland avenues on the west.

“For 20 years in Chicago, I’ve worked in not-for-profit and government,” Cassidy said on Sunday. “This is an opportunity to take it to the next level and an exciting challenge.”

Committeewoman and former State Sen. Carol Ronen says the decision was not an easy one but that the vote for Cassidy was unanimous.

“Kelly has a long record of advocacy with issues important to the people of the 14th District,” Ronen said. “Through the state’s attorney’s office, she has worked as a liaison in Springfield and understands how the legislature works and how to build coalitions.”

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Cassidy reports to Springfield on May 16. She knows there will be some “tough budget votes right out of the gate.”

“In our district, there’s a high level of social service organizations offering important services,” said Cassidy, 42. “They are threatened, and it’s my job to defend them.”

Cassidy said she will run in the March 2012 election

“I intend to serve the district as long as I can,” she said.

Another lesbian who was being considered for the appointment, Paula Basta, director of the Levy Center, also has announced her candidacy.

Cassidy is the third openly gay or lesbian member of the state House. State Rep. Deborah Mell (D-Chicago), the daughter of Ald. Richard Mell (33rd) and the sister-in-law of former Gov. Rod Blagojevich, is also openly lesbian, while state Rep. Greg Harris (D-Chicago) is openly gay.

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The Chicago Sun-Times contributed to this report, via the Sun-Times Media Wire