Joseph P. Kennedy III, the grandson of Robert F. Kennedy, is running for Congress.

Kennedy will seek the seat being left open by Rep. Barney Frank, D-Mass., a liberal who decided not to run for reelection as his district became more Republican, The Associated Press reported.

Kennedy, 31, a prosecutor in Massachusetts, is the first of his family's generation to seek office. His father served in Congress from 1987 to 1999.

In early January, John Walsh, the chairman of the Massachusetts Democrats, said in a statement that he was "excited" that Kennedy was considering a run for office.

Kennedy is expected to make his announcement on Thursday, the AP reported.