The S.E.I.U. does hope to somehow unionize throngs of fast-food workers, but those efforts may prove difficult given that most fast-food employees are scattered among thousands of different franchised restaurants. Moreover, the franchisees and fast-food chains are likely to mount a fierce battle against unionization.

The two-day convention, with 150 tables spread across the expo center’s floor, highlighted the campaign’s growth since November 2012, when 200 workers went on a one-day strike at 60 fast-food restaurants in New York. In its most recent strike in mid-May, workers walked out at restaurants in 150 cities nationwide, with solidarity protests held in 30 countries. The focus increasingly includes unionizing; the movement’s motto has become “$15 and a union.”

“My sense is there’s been a recognition on the part of the S.E.I.U. that to get the labor movement out of the very deep rut it’s in, it’s going to take more than an individual local organizing drive — that this is a moment to do a large-scale, high-visibility effort to alter the climate for labor,” said Janice R. Fine, a professor of labor relations at Rutgers University. “They have taken a sector like fast food where the conditions are well known: low wages, part-time hours, irregular hours, often no benefits.”

Image The Rev. William Barber addressed convention attendees. Credit... Nathan Weber for The New York Times

In a high-stakes move, fast-food workers, backed by the S.E.I.U., have brought several cases before the National Labor Relations Board, asking its general counsel to declare McDonald’s a joint employer of the restaurants run by its franchisees. If the labor board agrees, that would open the door for the S.E.I.U. to try to unionize not just three or five McDonald’s at a time, but dozens and perhaps hundreds.

Business groups oppose such a change. Mr. Spencer said that if the labor board adopted a broader interpretation, “then you make the corporation sit down at the table and talk” with the union. S.E.I.U. officials want McDonald’s and its franchisees to agree not to fight unionization efforts.

McDonald’s declined to comment on the weekend convention, held four miles from its headquarters. The main target of the movement’s attacks and unionization talk, McDonald’s says it provides competitive pay and benefits and opportunities for many workers. The National Restaurant Association has repeatedly criticized the call for a $15 fast-food wage, saying it would result in higher menu prices and cuts in employment.