Tokyo, Japan - Japan's Nippon Steel has hired former Secretary of State Mike Pompeo as an adviser to help with its proposed acquisition of US Steel, Kyodo News reported on Saturday.

The Japanese firm described Pompeo as being "respected by both" the Democratic and Republican camps, and as having "unparalleled insight into US geopolitical and security challenges," Kyodo said.

The company could not immediately be reached for comment.

Critics such as the United Steelworkers (USW) union see the deal as a threat to the industry in Pennsylvania, where US Steel is based, and the acquisition plan has become a political issue ahead of the 2024 presidential election.

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Both President Joe Biden and challenger Donald Trump have vowed to annul the deal as they compete for blue-collar votes, putting the transaction into limbo, probably until after the November poll at least.

The plan was announced in December 2023, with the Japanese steelmaker promising investments to keep Pennsylvania factories competitive with foreign producers, as well as newer "mini mills" in the American South that are less taxing on the environment.

Nippon Steel said in April that the proposed $14 billion takeover was going through an official US merger review following a report that the Justice Department had launched an antitrust investigation.

Pennsylvania is a key election battleground state, carried by Biden in 2020 but won by Trump in 2016.