Labor unions start to unify behind Kamala Harris. Here's why.

Randi Weingarten, head of the 1.7-million member American Federation of Teachers and a close ally of President Joe Biden, said she’d forgotten to bring something on stage with her at the union’s national convention in Houston, Texas on Monday.

“I don’t have tissues -- I should, unfortunately,” Weingarten said, before thanking Biden for his achievements. A day earlier, Weingarten had been among the first labor leaders to express support for the newly announced presidential candidacy of Vice President Kamala Harris.

"As you can imagine, my speech changed a lot in the last few days," Weingarten told the audience.

The sentiment reflects a bittersweet moment for organized labor, a key part of the Democratic coalition, which has indicated widespread support for Harris after remaining an ally of Biden up until his decision to step aside.

Within hours of Kamala’s candidacy, some of the nation’s largest unions offered an outright endorsement and others heaped Harris with praise, while acknowledging that an internal endorsement process must run its course.

The support owes to a perception of Harris as a labor ally and an heir apparent to Biden, as well as an acknowledgment of the difficulty of a shortened campaign in which unions are eager to turn toward defeating Trump, union leaders and labor experts told ABC News.

The onrush to back Harris is part of wider movement among Democratic Party leaders and elected officials that has made her an overwhelming front-runner for the nomination. No challenger has emerged as the party hurtles toward its convention next month.

“Labor unions are coalescing quickly around Kamala Harris,” Stuart Applebaum, president of the Retail, Wholesale, Department Store Union, which endorsed Harris, told ABC News. “We’re excited.”

The Service Employees International Union, whose 2 million members make it the nation’s largest private sector union, endorsed Harris on Sunday. So did Local 3000 of the United Food and Commercial Workers, a labor organization in the Pacific Northwest, which drew attention last week when it became the only Biden-aligned union to call for him to step aside.

On Monday, several other unions followed suit with endorsements, including the American Federation of Teachers.

Additional unions have released statements lauding Harris but stopping short of an outright endorsement as the organizations undergo a formal process for granting support.

AFL-CIO, the nation’s largest labor organization, which boasts 60 affiliate unions with 12.5 million members, issued a statement on Sunday speaking of Harris in favorable terms but remains in the midst of its endorsement process.

“It’s safe to say things are moving pretty quickly,” Steve Smith, deputy director of public affairs at AFL-CIO, told ABC News, noting that several of the affiliate unions had endorsed Harris. “She has close ties to many, many unions.”

The United Auto Workers, an influential union in key swing states, will likely convene its international executive board to discuss a possible endorsement, a union official said. In a statement, the official called Harris an “ally” and a “champion” for workers. UAW had previously endorsed Biden.

Still, at least one Biden-aligned labor leader voiced reluctance to join his peers in backing Harris. John Samuelsen, president of the Transport Workers Union, described the public push to remove Biden from the Democratic ticket as a “betrayal.”

“They did it in a way that tarnishes Biden and I don’t like it,” Samuelsen told ABC News.

Samuelsen said he wants to wait to see who joins Harris as her vice presidential nominee, assuming she becomes the nominee.

“I don’t have a relationship with her,” he added. “I got to know Biden pretty well. I have faith in Biden that he wouldn’t screw transport sector workers. At this point, workers have been betrayed so many times by both parties that it doesn’t make any sense not to be as prudent as possible.”

The Teamsters, a 1.3-million member union whose President Sean O’Brien spoke at the Republican National Convention last week, has not released a statement about Harris. The union has not traditionally endorsed a candidate until after both major party conventions, a spokesperson told ABC News.

On the whole, unions have come out in support of Harris not only because of Biden’s perceived strength with organized labor but also the limited time remaining in the campaign, Johnnie Kallas, a professor of labor and employment relations at the University of Illinois, told ABC News.

“The reality is we’re three-and-half-months out from Election Day,” Kallas said. “If this was a conversation, it should have been had a year ago.”