After more than a month of speculation, Joseph P. Kennedy III is set to formally declare his candidacy for Congress Thursday morning.

Kennedy’s campaign said it will release a video statement announcing his bid early Thursday morning before beginning an all-day tour of the Fourth District.

“I’ve spoken to people from across the 4th Congressional District—from Newton to Fall River—who believe that Washington no longer works for them,” Kennedy said in a statement on Wednesday. “I will work hard to earn every vote and if elected bring the fight for fairness to the U.S. Congress.”

Kennedy announced last month that he would leave the Middlesex County District Attorney’s office to pursue a possible bid for the Fourth District seat held by retiring Congressman Barney Frank ’61-’62.

Kennedy has gained momentum in recent days even though it remained unclear when he would formally enter the race.

A poll late last week showed Kennedy leading his chief Republican challenger, Sean Bielat, two to one. On Sunday, Kennedy received the endorsement of the Massachusetts AFL-CIO, the state’s largest labor union.

Kennedy’s campaign appears to be using some of the tactics employed by Harvard Law School professor and U.S. Senate candidate Elizabeth Warren.

Warren also announced her candidacy by video, followed by several pit stops throughout Massachusetts. Both campaigns are being advised by Democratic consultant Doug Rubin.

Political analysts said the nature of the announcement and the length of Kennedy’s exploratory period raise red flags about the 31-year-old’s readiness to enter the race.

“I think that they want to keep it very controlled,” Republican political consultant Todd Domke said. “They don’t want to open him up to a lot of questions.”

Democratic political consultant Michael Goldman said that the campaign’s caution may be caused by the pressure of the Kennedy legacy.

“When your name is Kennedy, the expectation is that by osmosis you know the answers to all the questions,” Goldman said. These expectations, he added, could hurt an unprepared candidate.

Kennedy is the son of former Mass. Congressman Joseph P. Kennedy Jr. and the grandson of former Attorney General Robert F. Kennedy. His announcement Thursday marks the first time a member of the family has run for office since the death of Edward M. Kennedy ’54-’56 in 2009.

The Kennedy name will likely give the campaign a boost in fundraising and media attention, analysts have said, which may translate into votes.

Kennedy will attend three major fundraisers on his behalf in Washington, D.C., next week, according to a report in the Boston Globe.

Even though he is little-known among voters, Kennedy is considered the de facto early frontrunner, analysts said.

Bielat is considered to be Kennedy’s chief opponent after a host of other candidates dropped their bids in the wake of Kennedy’s announcement of a potential run.

“What I’m not sure any of us could have expected was how quickly the field cleared out,” said former Boston City Councillor Lawrence S. DiCara ’71.

Kennedy is a graduate of Stanford and Harvard Law School. He served in the Peace Corps for two years before returning to Massachusetts to work as a prosecutor.

—Staff writer Nicholas P. Fandos can be reached at nicholasfandos@college.harvard.edu.