Former Gov. Deval Patrick of Massachusetts officially entered the presidential race on Thursday, adding an 18th candidate and an 11th-hour twist to a turbulent Democratic primary with less than three months to go before the Iowa caucuses.
Mr. Patrick’s announcement, which he had signaled this week, came in the form of a video he released early Thursday morning. In it, he offered an explanation of why he was joining the race now after having passed on a bid for the White House a year ago, saying he was running for people who “feel left out” and want a future “not built by somebody better than you, not built for you, but built with you.”
“I admire and respect the candidates in the Democratic field,” he said. “They bring a richness of ideas and experience and a depth of character that makes me proud to be a Democrat. But if the character of the candidates is an issue in every election, this time is about the character of the country. This time is about whether the day after the election, America will keep her promises. This time is about more than removing an unpopular and divisive leader, as important as that is, but about delivering instead for you.”
Mr. Patrick, 63, who served two terms as governor and is one of the highest-profile black leaders in the Democratic Party, is expected to appear on “CBS This Morning” later Thursday and offer additional details about his run.
Then he will head to New Hampshire’s State House to file paperwork to be on the primary ballot there — one day before the deadline — before moving on to California, Nevada, Iowa and South Carolina, according to a Democrat familiar with his plans.
His late entry to the race will present Mr. Patrick with an uphill climb to the nomination. He will start with zero campaign cash, little organization and none of the polling numbers he needs to qualify for a debate.
He has waited so long that nine other Democratic hopefuls have already come and gone, part of the biggest presidential field in modern political history. He will have much less time to work with than his close friend Barack Obama did in his first presidential campaign, when he started his run 11 months before the Iowa caucuses.
And yet, Mr. Patrick may not be the last person to enter the contest; Michael R. Bloomberg, the former mayor of New York, has also taken steps toward entering the Democratic primary after initially ruling it out.
The moves by both men reflect unease among some Democrats around the current state of the race and underscore the fact that no candidate has yet emerged as a dominant force. Former Vice President Joseph R. Biden Jr. has been a mainstay at or near the top of the polls but has not pulled away from leading progressives like Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont, or more moderate alternatives such as Pete Buttigieg, the mayor of South Bend, Ind.
Still, polls have shown that Democratic voters have been mostly satisfied with their options in the field.
In recent days, as Mr. Patrick has begun to disclose his plans, he has told advisers that he hopes to appeal to a wide swath of voters, bridging ideological and demographic divisions that have cleaved the party in the primary campaign so far.
Mr. Patrick grew up poor on Chicago’s South Side, went to Harvard for undergraduate studies and law school and then worked for the N.A.A.C.P. Legal Defense and Educational Fund. While there, he sued Bill Clinton, then the governor of Arkansas, in a voting case. He later worked for President Clinton’s Justice Department.
After turning his career to the private sector — he worked as general counsel at Texaco and later took a top position at Coca-Cola — he won the governorship in 2006 as a political outsider with grass-roots support from progressives. After leaving office in 2015, he joined Bain Capital, the private equity firm that Senator Mitt Romney of Utah co-founded, an association that started to draw some criticism from liberals as Mr. Patrick moved toward a presidential run.
Mr. Patrick told The Boston Globe on Wednesday night that he had resigned from Bain, effective that day. He also said he had spoken with Mr. Obama on Wednesday and that the former president had offered him advice.
In his announcement video, Mr. Patrick offered a hint of what he hoped would be a unifying message in the weeks ahead.
“We will build as we climb, to welcome other teachers and learners, other seekers of a better way and builders of a better future,” he said. “This won’t be easy, and it shouldn’t be.”
Jonathan Martin contributed reporting.