Workers at a rural Georgia factory that builds electric school buses under generous federal subsidies voted to unionize on Friday, handing organized labor and Democrats a surprise victory in their hopes to turn huge new infusions of money from Washington into a union beachhead in the Deep South.

The company, Blue Bird in Fort Valley, Ga., may lack the cachet of Amazon or the ubiquity of Starbucks, two other corporations that have attracted union attention. But the 697-to-435 vote by Blue Bird’s workers to join the United Steelworkers was the first significant organizing election at a factory receiving major federal funding under legislation signed by President Biden.

“This is just a bellwether for the future, particularly in the South, where working people have been ignored,” Liz Shuler, president of the A.F.L.-C.I.O., said Friday evening after the vote. “We are now in a place where we have the investments coming in and a strategy for lifting up wages and protections for a good high-road future.”

The three bills making up that investment include a $1 trillion infrastructure package, a $280 billion measure to rekindle a domestic semiconductor industry and the Inflation Reduction Act, which included $370 billion for clean energy to combat climate change.

Each of the bills included language to help unions expand their membership, and Blue Bird’s management, which opposed the union drive, had to contend with the Democrats’ subtle assistance to the Steelworkers.

Blue Bird stands to benefit from the new federal funds. Last year, it hailed the $500 million that the Biden administration was providing through the infrastructure bill for the replacement of diesel-powered school buses with zero- and low-emission buses. Georgia school systems alone will get $51.1 million to buy new electric buses, but Blue Bird sells its buses across the country. Still more money will come through the Inflation Reduction Act, another law praised by the company.

But that money came with strings attached — strings that subtly tilted the playing field toward the union. Just two weeks ago, for instance, the Environmental Protection Agency, which administers the Clean School Bus Program, pushed a demand on all recipients of federal subsidies to detail the health insurance, paid leave, retirement and other benefits they were offering their workers.

They also required the companies to have “committed to remain neutral in any organizing campaign and/or to voluntarily recognize a union based on a show of majority support.” And under the rules of the infrastructure bill, no federal money may to be used to thwart a union election.

The Steelworkers union used the rules to its advantage. In late April, it filed multiple unfair labor practice charges against Blue Bird’s management, citing $40 million in rebates the company had received from the E.P.A., which stipulated that those funds could not be used for anti-union activity.

“The rules say if workers want a union, you can’t use any money to hire anti-union law firms, or use people to scare workers,” Daniel Flippo, director of the Steelworkers district that covers the Southeast, said before the vote. “I’m convinced Blue Bird has done that.”

Politicians also got involved. Georgia’s two Democratic senators and southwestern Georgia’s Democratic House member also subtly nudged the plant’s management, in a union-hostile but politically pivotal state, to at least keep the election fair.

“I have been a longtime supporter of the USW and its efforts to improve labor conditions and living standards for workers in Georgia,” the Democratic congressman, Representative Sanford Bishop, wrote of the United Steelworkers in an open letter to Blue Bird workers. “I want to encourage you in your effort to exercise your rights granted by the National Labor Relations Act.”

Blue Bird’s management minimized such pressure in its public statements, even as it fought hard to beat back union organizers.

“Although we respect and support the right for employees to choose, we do not believe that Blue Bird is better served by injecting a labor union into our relationship with employees,” said Julianne Barclay, a spokeswoman for the company. “During the pending election campaign, we have voiced our opinion to our employees that a union is not in the best interest of the company or our employees.”

Friday’s union victory has the labor movement thinking big as the federal money continues to flow, and that could be good for Mr. Biden and other Democrats, especially in the pivotal state of Georgia.

“Workers at places like Blue Bird, in many ways, embody the future,” Mr. Flippo said after the vote, adding, “For too long, corporations cynically viewed the South as a place where they could suppress wages and working conditions because they believed they could keep workers from unionizing.”

The Blue Bird union shop, 1,400 workers strong, will be one of the biggest in the South, and union leaders said it could be a beachhead as they eyed new electric vehicle suppliers moving in — and potentially the biggest, most difficult targets: foreign electric vehicle makers like Hyundai, Mercedes-Benz and BMW, which have located in Georgia, Alabama and South Carolina in part to avoid unions.

“Companies move there for a reason — they want as smooth a path toward crushing unions as possible,” said Steve Smith, a national spokesman for the A.F.L.-C.I.O. “But we have federal money rolling in, a friendly administration and a chance to make inroads like we have never had before.”

The Blue Bird plant, which rises suddenly off a rural highway lined with peach and pecan orchards, has long made it a practice to hire less educated workers, some of whom have prison records and most of whom start at $16 or $17 an hour, said Alex Perkins, a main organizer for the United Steelworkers in Georgia.

A union was a tough sell for such vulnerable workers against a management that was fiercely opposed, organizers conceded. Coming off the last shift of the day on Thursday, most workers declined to speak on the record. A clutch of about a dozen workers stood on Friday at the Circle K gas station across the street from the plant in the predawn darkness, holding pro-union signs as the first workers arrived to cast ballots under the gaze of National Labor Relations Board monitors.

But Cynthia Harden, who has worked at the plant for five years and voted in favor of organizing, did talk about the pressure workers were under to vote against it. Slide shows on the voting process, which showed ballots marked “no,” said that the company could go broke if the union won, and there was a sudden appearance of food trucks at lunch and banners on the perimeter fence reading, “We Love Our Employees!”

“They’ve made some changes already, but if the union hadn’t started, nothing would have happened,” she said.

The letter that Georgia’s Democratic senators, Raphael Warnock and Jon Ossoff, wrote to Matt Stevenson, Blue Bird’s chief executive and president, was remarkably timid, praising the company for its cooperation and its well-paying jobs before “encouraging all involved, whatever their desired outcome, to make sure that the letter and the spirit of the National Labor Relations Act are followed.”

Mr. Perkins fumed at that tone, considering the work that unions had put in to help Mr. Warnock win re-election last year. “I won’t forget it next time,” he said.

Both senators declined requests to comment on the election.