The Internal Revenue Service is demanding a whopping $7 billion or more in back taxes from the world’s most profitable hedge fund, whose boss’s wealth and cyber savvy helped Donald Trump pole-vault into the White House. Suddenly, the government’s seven-year pursuit of Renaissance Technologies LLC is blanketed in political intrigue, now that the hedge fund’s reclusive, anti-establishment co-chief executive, Robert Mercer, has morphed into a political force who might be owed a big presidential favor. With Trump in the Oval Office, Mercer and his daughter Rebekah, who has become his public voice, seem armed with political firepower every which way...