United States Steel warned Wednesday it could shut its headquarters and factories in Pennsylvania, a key swing state in the upcoming election, if a takeover by Japan's Nippon Steel is blocked.

Both Kamala Harris and Donald Trump have come out against the sale as they campaign to win the White House, but US Steel planned a staff rally later Wednesday in support of the takeover.

"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," said company chief David Burritt.

In December, US Steel sealed a $14.9 billion agreement to sell itself to Japan's Nippon Steel, which has promised investments to keep Pennsylvania factories competitive with foreign producers and newer "mini mills" in the American South.

But the transaction, which has been denounced by the United Steelworkers (USW) union, has proven a tough sell politically with both the Democratic and Republican parties fighting against it.

On Monday, during a Labor Day event in Pittsburgh, Vice President Kamala Harris mirrored President Joe Biden's stance, declaring that US Steel "should remain American owned and American operated."

Former president Donald Trump has also vowed to block the deal, while his running mate J.D. Vance, has led congressional opposition to the takeover, describing domestic steel production as a national security priority.

US Steel has argued that the Nippon deal is needed to ensure sufficient investment in its Mon Valley plants in Pennsylvania, the earliest of which dates to 1875.

Nippon has promised to keep the plants open and invest $1.4 billion in USW-represented facilities through 2026 when the current labor contract expires.

The company has also vowed to keep US Steel's 1,000-worker office in downtown Pittsburgh.