Washington DC - President Joe Biden won the endorsement of the United Steelworkers (USW) union Wednesday, less than a week after saying he opposed the proposed sale of US Steel to Japan's Nippon Steel.

The planned $14.1 billion acquisition has become a hot political issue ahead of November's presidential election, with the union and lawmakers from both parties speaking up against the sale of an American manufacturing icon to a foreign owner.

"President Biden proved time and again during his first term that he stands with working families," said USW International president David McCall in a statement.

Biden has relentlessly courted the union vote as he competes with Republican opponent Donald Trump for working-class voters' support.

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Workers from key industries in swing states such as Pennsylvania and Michigan could shift the balance towards either party as the election draws close.

US Steel is based in Pittsburgh – in the battleground state of Pennsylvania – which Biden won in the 2020 election and will again battle Trump for this year.

Last week, Biden said it was vital for US Steel "to remain an American steel company that is domestically owned and operated," in a statement that was welcomed by USW.

In announcing its endorsement on Wednesday, the union highlighted Biden's landmark policies, such as the Inflation Reduction Act and what it dubbed "pro-worker legislation."

"Our nation has long needed this sort of sweeping investment. President Biden made it happen," said McCall, adding that Biden was also making good on his promise to create good, union jobs.

The union's backing comes after it surveyed members on their top priorities.

Winning over working-class voters was key to right-wing populist Trump's shock election win in 2016 and to Biden's in 2020.

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There is little doubt that in both camps, that bloc will be important again.

In January, Biden was also endorsed by the country's biggest car workers union, the United Auto Workers.

That backing could carry a lot of weight in Michigan, home to the US auto industry.

Biden appeared on a UAW picket line last year during a strike against the big three US car giants.