“With a field this massive, we’re in no rush to settle, and we aren’t interested in anything short of a full-throated, unapologetic advocate for the labor movement,” says John Weber, a spokesman for the AFL-CIO who offered this warning to the Democratic candidates: “We shouldn’t just be a part of your platform. Unionism should be the lens you use to look at the problems we’re facing and the tool you’ll use to build a fairer economy.”

Biden has been speaking the language of unions for years, as he’s cultivated his “Middle-Class Joe” image and pitched himself as an embodiment of the working class who can win the Rust Belt back for Democrats. In the lengthy run-up to Biden’s formal declaration for president, many of the public speeches he gave were to labor unions. And with Senator Sherrod Brown of Ohio forgoing a White House run, Biden has the longest and closest relationships with union leaders of anyone in the Democratic field.

“You can see that there is a reservoir of goodwill in the union movement for Joe Biden because of [his] decades of work understanding that the American dream is driven through the labor movement,” Randi Weingarten, the president of the American Federation of Teachers (AFT), told me by phone on Monday.

Weingarten said Biden was “the go-to person in the Obama White House” on labor issues, and said she used him as a back channel to the president during her highly public disputes with Education Secretary Arne Duncan. “Joe Biden was the one we would talk to to try to get to a fair hearing with Barack Obama,” Weingarten said.

Biden’s record on issues prioritized by the labor movement is not as pristine as he might want Democratic voters to think. He supported both the North American Free Trade Agreement and the Obama-negotiated Trans-Pacific Partnership, which unions loudly opposed. And his rivals Warren and Sanders have already started hammering him over his support for legislation, backed by the powerful Delaware-based credit-card companies, that made it harder for people to get out from under debt by declaring bankruptcy.

“Has he had missteps over his very long career? Of course. We all have,” Weingarten said. “But his heart and soul have been with growing the middle class and understanding that workers don’t get a fair deal unless they have a union.”

Despite Weingarten’s praise for Biden, the AFT isn’t close to making an endorsement, and she noted that candidates such as Sanders, Warren, and Harris, among others, “have had long relationships with labor, too.” That message of polite neutrality is one that other powerful unions are echoing at this early moment in the race.

“We’ve had a long relationship with Vice President Biden on lots of issues,” says Mary Kay Henry, the president of the Service Employees International Union (SEIU), citing health care as an example. But, Henry quickly added, “it’s similar to, frankly, the long-standing relationship we’ve had with many of the other Democratic candidates.”