WASHINGTON :Nippon Steel alleges the White House had "impermissible undue influence" over a national security review of its $14.9 billion bid for U.S. Steel and threatened legal action if the deal is blocked. The accusation was made in a Dec. 17 letter, signed by counsel for Nippon Steel and U.S. Steel and seen by Reuters, to the Committee on Foreign Investment in the United States (CFIUS). CFIUS, which reviews foreign acquisitions for national security risks, has a Monday deadline to approve