By Jon Herskovitz

AUSTIN, Texas (Reuters) - The district attorney for Dallas County, one of the most populous in the United States, resigned on Tuesday after battling personal issues that included three lengthy visits to treatment centers for depression since taking office in 2015.

Susan Hawk, a first-term Republican who had previously been a state judge, has publicly acknowledged seeking treatment for depression and prescription drug abuse.

Since being sworn into office in January 2015, Hawk, 46, has come under public scrutiny for her decision-making and behavior. After taking her post, she fired several top prosecutors and employees, causing problems for prosecutions in a county with some of the heaviest case loads in the country.

"It is with a heavy heart that I must tender my resignation ... It's been an honor and a privilege to serve this office and the citizens of Dallas County alongside you, but my health needs my undivided attention," she wrote in a letter to Texas Governor Greg Abbott.

Abbott, a Republican, will appoint Hawk's successor. The Democratic Party has been developing a power base in Dallas County, in the solidly Republican state.

In July 2015, Hawk disappeared from the district attorney’s office for more than two months, with her whereabouts unknown.

When she returned in October of that year, Hawk said she had sought treatment for depression at a Houston psychiatric hospital.

Earlier this year, Hawk prevailed in a legal battle against an ousted aide who unsuccessfully attempted to remove her from office.

"It is concerning that she has taken so long to recognize that she was unfit for the office," said Mark Haney, an attorney who fought for the aide seeking to oust Hawk.

(Reporting by Jon Herskovitz; Additional reporting by Marice Richter in Dallas; Editing by Tom Brown)