WASHINGTON — To be clear, no one in President Joe Biden’s White House would ever root for Donald Trump. To a person, they consider him an existential threat to the nation. But as they watched Trump open the contest for the Republican presidential nomination with a romp through Iowa, they also saw something else: a pathway to a second term. Biden’s best chance of winning reelection in the fall, in their view, is a rematch against Trump. The former president is so toxic, so polarizing that his pre