WASHINGTON (Reuters) - U.S. Steel warned Wednesday that a failure to merge with Nippon Steel would put thousands of U.S union jobs at risk and result in the American steelmaker largely pivoting away from its blast furnace facilities and potentially moving its headquarters.

The announcement comes as the deal faces strong bipartisan opposition with Vice President Kamala Harris on Monday saying she wants U.S. Steel to remain "American owned and operated". Her Republican rival Donald Trump has pledged to block the deal if elected.