When Darcel D. Clark left a Bronx housing project for Boston College, and later Howard University School of Law, she did not forget the struggles of those she grew up with. She once told her college roommate that if she could do something about the drug deals and the crimes in the Bronx, she would.

Now, three decades later, Justice Clark is poised to become the top prosecutor in the borough.

Justice Clark, 53, an associate justice of the New York State Supreme Court Appellate Division, was chosen last week by Bronx Democratic leaders to succeed the Bronx district attorney, Robert T. Johnson. If elected in November, Justice Clark — who does not yet have a Republican challenger in the predominantly Democratic borough — would become the first woman to hold the position.

Justice Clark had been considered a leading candidate for the post, though she is not well known outside legal circles. A former assistant district attorney who once worked for Mr. Johnson, she is seen as a close ally of Carl E. Heastie, the New York State Assembly speaker, the most powerful politician in the Bronx. When Mr. Heastie, a Democrat, was sworn in as speaker in February, he asked the justice to come to Albany to administer the oath.

Mr. Johnson is stepping down to seek a judgeship on the State Supreme Court, as part of a series of political moves engineered by Bronx Democratic leaders. During his tenure, the office has been criticized for a low conviction rate, a high percentage of dismissed cases and a crippling backlog that has left defendants and victims in limbo.