The Monongahela River winds through the tight Mon Valley south of Pittsburgh, Pennsylvania, creating a main artery in the nation’s industrial heart, where the steel and coal industries have driven the region’s economy and shaped political landscapes since the late 19th century.

In the weeks preceding the election, the region is once again playing an outsize role in determining the nation’s political future. A controversial Biden-Harris administration plan to kill Pittsburgh-based US Steel’s proposed sale to Japan’s Nippon Steel is viewed in part as an election-year strategy to shore up critical union support in a must-win swing state.

On the ground in and around the city, evidence suggests the move may just work – unions oppose the sale and the administration’s position is at the very least maintaining recent Democratic gains in the tug-of-war for swing voters in the nation’s steel capital.

Anecdotal evidence and polling point to Harris gaining momentum here.

“I’ve learned not to be comfortable with any election because we didn’t think Trump could win in 16 … but I think people are going to vote more common sense this year,” said Keli Vereb, a steelworker union rep and Lincoln borough council member.

Unusually in these fractious times, both presidential candidates oppose the deal, backing United Steelworkers International union members across the political spectrum who are determined to thwart a deal they see as a job killer that puts their pensions at risk.

Recent memories of supply chain issues have also hardened US resolve to protect vital industries such as steel.

Still, politics are omnipresent, and the deal undoubtedly will play a role in determining the next president. It comes eight years after blue-collar workers here defected from the Democratic party en masse when then candidate Hillary Clinton said during a debate that she would put coalminers out of business.

Some union leaders say the comment may have cost her Pennsylvania, which Donald Trump won by 0.7%. After four years of pro-labor policies from Joe Biden, the party has begun to win back some who left, and with Trump proposing to block the US Steel sale if he were elected, Democrats risk a 2016 repeat if it is allowed to proceed.

“Trump would pounce on them if they let [the sale] go,” said Allen George, a lifelong Democrat who worked in unions adjacent to the steel industry.

The companies are making a powerful argument that the deal is vital to US Steel’s survival. US Steel claims it will be forced to cut Pennsylvania jobs and move its headquarters out of Pittsburgh if Biden blocks Nippon’s $14.1bn bid, while it has promised to invest $2.4bn in its facilities if the sale goes through. The company’s “scorched earth” public relations campaign on the factory floors has at least some rank and file supporting the sale, said Bernie Hall, Pennsylvania director for USI.

“Some are scared and think: ‘We should just take this and live to fight another day,’ and that’s natural,” Hall said.

Many more, however, oppose the sale. The union’s contract is up in 23 months and they fear a Nippon-US Steel would cut jobs, or continue to send them to non-union states. They point to Nippon’s long history of “dumping” steel in the US, which has cratered prices and cost American jobs, and many fear the purchase is a ploy to continue the practice.

US Steel’s record of closing factories and failing to keep promises has generated a deep mistrust and disdain for the company, workers told the Guardian on a recent Monday afternoon outside the Harvey Wilner’s pub in West Mifflin, just south of Pittsburgh. They rattled off a list of facilities that have closed over the decades.

“Nippon can have at it,” said Barry Fez, who has worked in manufacturing in the region for decades, but, he says, in a few years he expects they will go back on their word.

But that sentiment is colliding with Wall Street and Beltway support for the deal. The latter argue that the administration’s protectionist plan would run counter to international trade norms because Japan is an ally and close economic partner.

The idea that trade decorum with Japan is more important than Pennsylvania union members’ security drew scoffs from some workers.

“And then they’ll wonder why they lost an election,” said Mike Gallagher, a retired union member.

‘They lie all the time’

Banking legend JP Morgan created US Steel in a mega-merger in 1901. It grew to be the largest US producer, employing more than 340,000 people at its second world war peak. Today, it is a shadow of its former self, has closed many of its Mon Valley facilities, and now employs about 4,000 people, although the company says it indirectly supports 11,000 jobs and generates $3.6bn in economic activity annually.

In the face of waning American steel power, the company has looked for a buyer, and many feel a US-Japan alliance makes sense in countering increasing Chinese domination of the industry.

But the union is opposed, and in Pennsylvania, 25% of the electorate is unionized, making it a formidable bloc intensely courted by both political parties.

Trump in January said he would stop the deal. Biden has said the same, including in a private meeting with steel workers in April, when the president insisted “US steel will stay US-owned”, according to Don Furko, president of Local 1557 in Clairton. “He said he ‘guarantees’ it.”

The administration’s decision on whether the deal should be blocked largely lies with the Committee on Foreign Investment in the United States, or CFIUS, which is made up of Biden’s cabinet members and other appointees. It can veto mergers and acquisitions it finds present a national security risk.

CFIUS was expected to issue an opinion on 21 September, but the administration punted until after the election. Union members say they aren’t worried.

“President Biden and Vice-President Harris have been pretty clear and they will follow through,” Hall said.

Harris has got the message: “US Steel should remain American-owned and American-operated,” she told a rally in Pittsburgh earlier this month.

David Burritt, the CEO of US Steel, has warned of consequences if the deal is blocked. He says the company would “largely pivot away” from its blast furnace production in the region, and move its headquarters out of Pittsburgh.

“We want elected leaders and other key decision makers to recognize the benefits of the deal as well as the unavoidable consequences if the deal fails,” Burritt said last month.

That threat has further inflamed tensions. Furko said it reminds him of his young son flipping over the Monopoly board when he loses: “That’s really what’s going on here – if this deal doesn’t go through, then they’re going to flip over the Monopoly board.”

Asked about US Steel’s claims that it will revitalize the region if the sale goes through, workers told the Guardian that there are no guarantees that the investment will be in Mon Valley. People would be “foolish” to believe that, Vereb said.

That was echoed outside the Wilner’s pub. Fez recalled the pub’s heyday, when “you couldn’t get in there at 7am because it was so packed”, and the floor was littered with quarter wrappers from the slot machines.

On a Monday afternoon around shift change time, a group of about a dozen retirees sat around the bar. They blamed US Steel for the region’s slowdown, and while they say they do not expect Biden or Trump to save the city, they have even less confidence that US Steel and a Japanese company will turn it around.

“They lie all the time, and I don’t trust either one of them,” said Jack, a retiree who worked for US Steel for more than 30 years, who declined to use his last name.

‘He gets credit for that’

The political price that the Biden-Harris administration could pay for allowing the deal to go through can be seen in the 2016 election’s wake.

Before 2016, the region was largely Democratic. But when Clinton made the comment about the clean energy industry putting coalminers and barons out of business, “Things turned on a dime,” Vereb said. Her borough of 900 was once about 80% Democrats. It’s now about 75% Republican, she estimates.

About 75% of those working at US Steel’s Clairton Mill Works, several union leaders estimate, support Trump, and there is little Democrats can do to win back many of them.

The situation is also complicated by US Steel’s intense campaign to convince workers that the sale will save their jobs. The company sends regular emails, holds meetings, takes out ads in newspapers and makes their case to reporters.

“They say: ‘If you don’t support us, then we’re gonna shut this place down, and if that happens you can thank your union leadership,’” said Rob Hutchison, president of Local 1219. “When [rank and file] have that threat in their face eight to 12 hours per day, then it starts to become something they think about.”

That also presents another political risk: if the Biden-Harris administration were to block the deal, and US Steel shuts down a plant, Democrats may again lose some voters.

However, so far, the controversial move seems to be paying dividends.

“I don’t know if the average Joe is thinking about CFIUS or is that in the weeds, but I think from a macro level, people see it, that it’s Biden supporting the union workers, and he gets credit for that,” Hall said.