Deval Patrick has become the latest US Democrat to join the crowded race for the 2020 presidential election.
The two-time former governor of Massachusetts announced his candidacy on Thursday.
He joins 17 other Democrats still in the race, including former Vice-President Joe Biden and Senators Elizabeth Warren and Bernie Sanders.
His run starts just three months before the first votes are cast in the Iowa caucuses and New Hampshire primary.
Mr Patrick launched his campaign in an online video, saying the election was about more than President Donald Trump: "This time is about more than removing an unpopular and divisive leader - as important as that is - but about delivering instead for you."
Serving from 2007-2015, Mr Patrick, 63, made history as the first black governor of Massachusetts and one of the first in any US state.
Most recently he was a managing director for private equity firm Bain Capital.
The former federal prosecutor also has close ties to former President Barack Obama.
Just when it appeared the Democratic presidential field was shrinking in earnest, it starts expanding again.
First, former New York Mayor Michael Bloomberg began motioning toward a campaign. Now Deval Patrick has jumped in with both feet.
The former Massachusetts governor has a tall task ahead of him if he wants to seriously compete for his party's nomination. Unlike Mr Bloomberg, he does not have a seemingly bottomless personal fortune to draw on for campaign expenses. Fundraising is one of the most important components of a successful presidential bid, and Mr Patrick will start at zero with only a few months until the primaries begin.
It appears Mr Patrick will count on his personal charisma and a focus on early voting New Hampshire, which borders his home state, to jump-start his campaign. He is pitching himself as an optimistic mainstream Democrat who can unite the country after the divisive Trump presidency. And he's a charismatic African-American candidate who counts the Obamas as close friends.
Mr Patrick's last-minute entry reverses his decision last year not to run, citing the "cruelty" of the elections process.
He has already missed the deadlines to file for the state primaries in Alabama and Arkansas. But some say he could have a boost in the New Hampshire primary due to serving as governor in neighbouring Massachusetts.
It is also unclear if Mr Patrick will get enough support and funding to qualify for a presidential debate.
His announcement comes amid speculation that former New York City Mayor Michael Bloomberg may also be joining the crowded Democratic field. Mr Bloomberg, 77, filed primary paperwork in two states, but has not formally confirmed his candidacy.
Who will take on Trump in 2020?
Mr Patrick joins an already crowded field vying to take on President Trump in 2020.