‘I want to become the 55th mayor of the City of Boston’

Ending months of speculation, City Councilor Tito Jackson will announce Thursday that he is jumping into this year’s mayoral race, in what he vows will be a vigorous campaign against his friend and former ally, Mayor Martin J. Walsh.

The 41-year-old Roxbury councilor, who will announce his mayoral candidacy at 2 p.m. at Dudley Square’s Haley House, is setting the stage for an uphill but competitive contest later this year. Although the field of candidates is not set, Jackson is hoping to become the first viable African-American candidate on the November mayoral ballot in more than three decades — since the candidacy of former state representative Mel King.

In an interview with the Globe this week, Jackson outlined his intentions, said he would be a stronger leader than Walsh, and laid out the case he plans to make to voters. He said his campaign will focus on the city’s struggling middle class and issues such as public education, affordable housing, and the staggering income inequality found through the city.

“I want to become the 55th mayor of the City of Boston to ensure that the city on the hill that has been welcoming and open to so many families . . . remains the city for middle- and working-class people,’’ Jackson said. “We are a city that has lost our way.”

With Jackson in the race, the sleepy election that many had anticipated will likely become a lively battle between two progressives vying for critical voting blocs, such as the black community, which was crucial to Walsh’s 2013 victory.

Walsh has enjoyed strong popularity throughout the city but has had setbacks in his administration, such as the failed Olympics and IndyCar race bids. A federal investigation in the Walsh administration also looms.

In a Wednesday interview, Walsh said he is staying focused on his job as mayor of the city, which he said has seen unprecedented growth. He said he will continue to work to improve the public school system, reform city government, and make Boston safe.

He would not respond to Jackson’s criticisms, saying he will not get into politics until candidates file the proper paperwork in April. Candidates can pull papers from the city’s Election Department between April 19 and May 16. Mayoral candidates must submit 3,000 signatures by May 23.

“I’m not going to get into the politics today,’’ Walsh said. “I don’t have the luxury of taking shots at people. My job is to get the city budget done, move forward, and . . . continue some of the great things we’ve done in our city. We have work to do in a lot of different areas.”

Jackson begins the race with a clear disadvantage. No incumbent has lost a Boston mayoral race since 1949. Historically, black candidates have been hindered by low voter turnout, a lack of money, and inability to match the incumbent. Jackson reported nearly $65,000 in the bank earlier this month, records show, compared to Walsh’s $3.6 million.

The councilor said his campaign raised nearly $88,000 in December. State records show he raised $57,000 during that time.

Jackson said becoming mayor has been on his mind for many years. He considered running in 2013, but personal foreclosure troubles kept him out of the race. He kept his house and rallied behind Walsh during that race, hopeful, he said, that the labor leader would become the people’s champion.

“I worked hard for him,’’ Jackson said. “I was really hopeful that this mayor was going to be the good pilot and that he would take the reins and make sure that everybody sits at the table. . . . But I am disappointed.”

Jackson said the tipping point came during the city’s failed Olympics bid, which the councilor said would have cost taxpayers billions. It made him question what else the administration was willing to risk.

“That showed me that [his] GPS was off, that there was a lack of a steady hand when it came to our future of this city,’’ Jackson said.

Jackson, seen as one of Boston’s more charismatic public figures, said he has been praying over whether to run for mayor and finally heeded “a calling” to help others the way he has been helped in throughout his life.

He brought on a Kentucky consultant, LA Harris & Associates, to boost his fund-raising; hired one of President Obama’s former political strategists, Bill Hyers, as an adviser; and spent $15,000 on a professional campaign video.

Jackson, in an hourlong Globe interview, shrugged off critics who say he lacks the money, ground game, and credibility to unseat Walsh. In response, Jackson’s supporters cite the improbable victories of the first campaigns of Obama and former governor Deval Patrick as proof that anything is possible.

Jackson challenged his critics to look at his record as an advocate for public education during proposed budget cuts. He helped lead the opposition to a November ballot measure to lift the cap on charter schools and heads the council’s Education Committee. He also has pushed for development that does not displace residents, been a shoulder for families grieving after violent crime, and joined with residents in their fight to keep their homes or get a job, he and supporters say.

“Let my work speak for me,’’ said Jackson, whose community fund also handed out thousands of turkeys over the past six Thanksgivings.

Jackson also criticized Walsh for his slow pace of action on race and gender issues, as well as what he sees as Walsh’s failure to improve the lives of residents, particularly in Roxbury, East Boston, and the South End.

This isn’t the first time Jackson has clashed with Walsh. The councilor was among the first to support giving Boston police body cameras; Walsh initially opposed them. The two were on opposite sides on legalizing recreational marijuana. And Jackson slammed the mayor over the controversial closing of the Long Island Bridge, which at the time led to the city’s only homeless shelter.

Jackson, a towering 6-foot-2 bachelor, was born on April 11, 1975, to a 13-year-old girl who had been sexually assaulted. After months in foster care, he was adopted by a Grove Hall couple, Rosa and Herb Jackson. They brought him to their Schuyler Street home — where he still resides — to live with their three children and later adopted three more children, the councilor said.

Their community was one where everyone took care of one another. A judge lived across the street, and a neighbor who owned an ice cream truck gave Jackson his first job.

Herb Jackson, now deceased, was a prominent community organizer at Greater Roxbury Workers Association who pushed for blacks and Latinos in Boston to have a decent wage in the building trades. Rosa Jackson ran a day care center called Listen, Look, and Learn.

Jackson went to Brookline High School and earned a degree in history from the University of New Hampshire. He spent more than a decade in pharmaceutical sales and marketing before serving on the economic development team in the Patrick administration. In 2009, Jackson lost a bid for councilor-at-large. He also became political director during the Patrick’s successful 2010 reelection campaign.

In 2011, Jackson won a special election to represent District 7, which along with Roxbury includes the South End.

In 2014, as debate swirled around whether councilors should give themselves a pay raise, Jackson spoke out on the council floor about the financial sacrifices he and his colleagues made to run for office.

“In my mind, public service shouldn’t be a subscription to poverty,’’ he said at the time.

He now says he regrets the comment.

Jackson told his council colleagues and the mayor of his intention to run Wednesday. He told his mother during his visit to her home in Jacksonville, Fla., around Christmas, as the two watched “The Golden Girls.”

She enveloped him in her arms, he recalled, and urged him to pray.