In the video, Patrick touts his political experience as governor and frames his path from Chicago's South Side to the Massachusetts State House as an example of the American dream.

"In a spirit of profound gratitude for all the country has given to me and with a determination to build a better, sustainable, more inclusive American dream for the next generation, I am today announcing my candidacy for president of the United States," Patrick says in the video.

Patrick plans to head to New Hampshire later Thursday to file for the state's first-in-the-nation primary, according to a source close to him.

Patrick's decision to run could not be more last-minute, according to one source, who said: "This is a long-shot, but all presidential campaigns are long-shots."

Patrick was considering running for president last year, but decided not to jump in out of concern for his family, calling the election process "cruel."

Alex Goldstein, who worked for Patrick for 10 years including serving as his press secretary, said he believes that whatever concerns the former governor had about his family's well-being have been resolved.

"I know the that the first and utmost priority was his family," Goldstein said. "I can feel pretty confident that once the family felt that it was okay [for Patrick to run], that would be a game changer for him."

Goldstein says Patrick will bring a powerful voice to the presidential campaign.

"Patrick is not only a spectacular orator, but he also feels this stuff deeply within him," Goldstein said.

But he also acknowledges that getting into the race this late will be a daunting challenge.

"Governor Patrick knows that he's probably not going to win this," Goldstein said. "But everything I know about him tells me that when he feels called to service, he's going to answer the call."

Patrick faces a number of challenges: many of his longtime aides are already committed to other candidates; he'll need to raise a lot of money quickly; he will be competing against candidates who have been on the ground for almost a year and who have extensive campaigns set up in the four states that vote first (Iowa, New Hampshire, Nevada and South Carolina). In addition, Patrick won't be able to participate in the Democratic debate next week, and probably won't qualify for the one next month. Finally, due to his late entry into the race, he will have to contend with a compressed the time frame.

"The New Hampshire primary is only 90 days away," said Mary Anne Marsh, a Democratic consultant in Boston. "That is followed a week later by the Nevada caucuses; a week after that, the South Carolina primary, and then a week after that is Super Tuesday. On all measures, it is nearly impossible to do."

Patrick served two terms as governor of Massachusetts, and was the state's first African-American chief executive. He is close to and admired by former President Barack Obama. He is also politically gifted and ran a memorable grassroots campaign during his first successful run for governor. But those skills might not be as effective in a national presidential race that stretches across a wide continent.

In addition, his competitors have already spent months introducing themselves to voters in school gymnasiums, theaters and town halls from Manchester to Des Moines. Elizabeth Warren, who registered Wednesday for the New Hampshire primary, has traveled to the state 22 times since she entered the race 11 months ago.

Among the questions facing Patrick is the most basic: why is he running?

According to the New York Times, which was the first to report earlier this week that he was considering a presidential run, Patrick told party leaders that he does not think anyone in the Democratic field has established real momentum, and he believes that there is room for a candidate who can unite liberal and moderate Democrats.

Goldstein says that while there are some "spectacular" Democrats already running, Patrick has a way of "injecting empathy into the conversation ... that will add value to the [presidential campaign.]"

As an example, Goldstein described traveling to South Carolina with his former boss last fall to help Democratic candidates before the midterm elections. They ended up at Mother Emmanuel Church in Charleston, the day after a gunman opened fire at the Tree of Life Synagogue in Pittsburgh.

"It was a moment of unbelievable tension and emotion," Goldstein said. "To see him with tears in his eyes on the pulpit as he addressed that congregation, it was in that moment when I said to myself, 'Oh my god, he's actually going to run, and he can actually win.'"

Patrick is joining a crowded Democratic field that includes Massachusetts' own U.S. Sen. Elizabeth Warren, who is among the top tier of candidates. In New Hampshire yesterday, Warren, who knows Patrick well, we asked if his entry into the race would complicate her own campaign.

"No," she said.

She was also asked about the critique by some that the current Democratic field isn't strong enough.

"I'm out there talking to people every single day about an America that works great for those at the top, but just isn't working for most everybody else," Warren said. "And whenever I do that, there are lots of folks who start to nod. And it's not just Democrats. It's Democrats, independents and Republicans who get it."

Indeed, if Patrick is hoping to be welcomed by legions of Democratic voters looking for a savior, he might be disappointed.

"Polls of primary voters indicate that they're pretty happy with the current crop of candidates," said Peter Ubertacio, a political scientist and dean of the School of Arts and Sciences at Stonehill College.

"There's been no real concern except for whispers among well-funded donors. And that's very different than the concerns of the primary voters."

Ubertacio says that many Democratic Primary voters might be put off by the suggestion that they need to be saved by someone who hasn't been doing the hard work of campaigning over the past many months. He says at this late date, Patrick's eloquence might not be enough to make his case.

This story comes from the New England News Collaborative, eight public media companies, including The Public's Radio, coming together to tell the story of a changing region, with support from the Corporation for Public Broadcasting.