The giant deal – worth 15 billion dollars – has been approved by the shareholders but met with scrutiny from the political side.
Biden has expressed doubts on economic and security policy grounds, while President-elect Donald Trump has clearly distanced himself, saying he intends to stop the merger.
The US steelworkers union is also skeptical and has expressed concerns that Nippon Steel will use the new joint venture to replace US-made steel with foreign steel.
Biden now has 15 days to make the announcement, according to a White House spokesperson.