Mr. Patrick began his campaign on Nov. 14, reversing his decision a year earlier not to run. By the time he entered, other candidates had a huge head start. Four debates had already passed.

He cast himself as the candidate best suited to unite moderates and progressives amid worries that none of the contenders already in the field would be able to build a coalition capable of defeating President Trump. His stump speeches drew from his compelling life story — he grew up in poverty on Chicago’s South Side before ascending to the upper echelons of business and politics.

But Mr. Patrick also came to the campaign with some potential baggage, given a growing progressive movement in the Democratic Party that is deeply suspicious of big business. His career included positions as corporate counsel at Texaco and, more recently, as a managing director at Bain Capital, a private equity firm.

And, unlike Michael R. Bloomberg, the billionaire former mayor of New York City who was also a latecomer to the race, Mr. Patrick did not have enough of a personal fortune to finance his own campaign. There was also little evidence that he had a donor base large enough to make up for months of lost fund-raising.

In his message to supporters on Wednesday, Mr. Patrick expressed irritation at the focus on the timing of his campaign.

“Many in the media have noted that I entered the race ‘late,’” he said. “As a direct and limiting consequence, I’ve met many people on the campaign trail who lament how they wished I had entered the race sooner. As I hope you know, I entered this race when I could, and not a moment before I should have. More importantly, I entered the race months before anyone had cast a vote.”