"The key is a psychological model for identifying voters that can “determine the personality of every single adult in the United States of America,” said Alexander Nix, chief executive of Cambridge Analytica. The little-known company, which has operated in the United States for four years, opened its office here only a month ago and is clearly at the center of Trump’s quest for a last-minute comeback against Democrat Hillary Clinton. New federal filings show the campaign’s payments to the firm ballooning from $250,000 in August to $5 million in September. The reliance on Cambridge reflects a recognition by Trump’s campaign...