“I’m not running,” he said at an event in California over the weekend, “to be president of the Democrats. I am running to be president of the United States. There’s a difference.”

But his decision to enter the race might be more about Cory Booker, Julián Castro and Kamala Harris. The campaigns of all three appear in tatters, with single-digit poll numbers and empty campaign coffers. Barring a significant turnaround, all might be finished before the New Hampshire primaries.

Fourteen years ago, Mr. Patrick’s egotistic presumption sounded equally implausible. And then, he got on the stump.

Over months on the trail, this corporate boardroom professional from the South Side of Chicago recruited an army of devotees, ranging from young idealistic progressives to converted veterans of Massachusetts machine politics.

One cried on the phone with me this week, explaining what Mr. Patrick meant to him.

With most of the political establishment against him, and running third among Democrats in polls, Mr. Patrick stood in front of roughly 4,500 party delegates in Worcester. His 10-minute speech urging “hope” — later liberally borrowed by his friend Barack Obama — electrified the crowd. People removed his opponents’ pins from their lapels as he spoke.

He won nearly 60 percent of delegates’ votes and went on to win the primary and general elections.

He was similarly counted out for re-election in 2010 — a national Republican wave election during the Great Recession, in a state that earlier in the year had famously elected Scott Brown in a special election over the Democrat Martha Coakley.

At the time of Mr. Brown’s election, Mr. Patrick had a 22 percent approval rating, and struggled to top 30 percent in polls against the Republican and Independent candidates.