Montgomery County State's Attorney Andrew L. Sonner announced this week that he will seek a third term.

Meanwhile, Daniel J. Cassidy, an attorney for the Montgomery County government, is expected to file tomorrow for the same office and to announce his campaign plans Sunday at a fundraiser at the Irish Inn in White Oak.

Both Sonner and Cassidy are Democrats, and both grew up in Montgomery County.

"Basically I'm running as an independent without the support of the Democratic Central Committee," said the 44-year-old Sonner. "That is, I won't submit my name to the Central Committee.

"There are still opportunities here to do exciting work. We have developed a reputation for being firm but very fair."

Sonner was deputy state's attorney from 1966 to 1970 and was in private practice from 1964 to 1966. He taught American history at Walter Johnson High School for six years while attending American University Law School at night.

Sonner said on advantage of being state's attorney was that it allowed to stay involved with community affairs. He note that he kept contact with the community he serves through his membership on several county commissions.

"Cassidy, 29, said he would like to set up a nine-member criminal justice advisory commission of civic leaders and community residents if he is elected state's attorney. The commission would hold open meetings on crime problems in the different areas of the county.

"The state's attorney has an enormous amount of discretion in how he handles criminal cases," said Cassidy. "We've had one individual who has decided priorities in the criminal justice system. We have not had any input from the community. Their values should be taken into account."

Cassidy graduated from the University of Baltimore Law School in December 1972. He immediately went to work as an assistant state's attorney in Prince George's County.

"I tried 107 jury trials, so I have extensive trial experience," he said. Cassidy began working for the Montgomery County government as a lawyer in 1976.