“It’s so fluid,” she said about the race. “At this point, it would be disingenuous for me to suggest that there is this magical date when we will know that it’s time.”

Several of the unions have held, or are planning to hold , candidate forums, including the N.E.A., which held one in Houston in July during its annual meeting that drew 10 candidates, and Afscme, which convened one in Las Vegas in early August that drew 19.

The S.E.I.U., one of the largest and most politically powerful unions in the country, has laid out specific criteria for candidates seeking its endorsement , including that they must support legislation to raise the minimum wage to $15 an hour and develop an economic plan that includes provisions to help workers unionize. The union has been in direct contact with many of the campaigns and is providing feedback on their policies for organized labor if they seek it.

The emphasis on labor in the 2020 campaign has at times tripped up even its staunchest supporters. Mr. Sanders, whose campaign has unionized, recently faced criticism from some of his campaign workers for failing to pay them the equivalent of a $15-an-hour minimum wage because they were working more hours than they had expected. His campaign resolved the issue by raising their salaries and limiting the number of hours some staff members could work.

Hotel spending by some of the leading candidates has also come under scrutiny. HuffPost reported in August that Mr. Buttigieg’s campaign had patronized nonunion hotels in several big cities , while the campaigns of Ms. Warren and Ms. Harris spent money at hotels that have been the subject of labor disputes.