Barbara A. Mikulski, a legend in Maryland Democratic politics and the state’s first female senator, endorsed former judge Katie Curran O’Malley in the competitive primary race to be attorney general. Mikulski said in a statement that she has known O’Malley for decades and believes “she is uniquely qualified” to become the state’s first female attorney general. The closely watched contest pits former allies against each other to succeed Attorney General Brian E. Frosh (D), who is retiring. Maryland has not elected a Republican attorney general since 1952.