Elise Amendola / AP
Former Mass. Rep. Joseph P. Kennedy II, left, hugs his son, Joseph P. Kennedy III during a campaign event for Martha Coakley in 2010. The younger Kennedy is expected to announce his candidacy for Congress on Thursday.
Joseph Kennedy III said Wednesday he's formally jumping into the race for the congressional seat now held by retiring U.S. Rep. Barney Frank.
Kennedy, the son of former U.S. Rep. Joseph Kennedy II and a grandson of the late Robert F. Kennedy, said he would work hard to earn every vote and if elected would bring a "fight for fairness to the U.S. Congress."
"I believe this country was founded on a simple idea — that every person deserves to be treated fairly, by each other and by their government, but that's not happening in America anymore," Kennedy, a Democrat, said ahead of an announcement scheduled for Thursday.
The campaign source told NBC News that Kennedy would make a video announcement in the morning, followed by a half dozen events throughout the district.
Kennedy recently moved from Cambridge to Brookline, part of the state's newly redrawn 4th Congressional District. The Kennedy family has deep ties to the Boston suburb.
Kennedy announced last month that he was forming an exploratory committee to look at a possible run for the seat.
At least two other Democrats and two Republicans are considering runs.
Kelly O'Donnell, NBC's Capitol Hill correspondent, and The Associated Press contributed to this report.