Joseph R. Biden Jr. has taken a commanding lead over President Trump in the 2020 race, building a wide advantage among women and nonwhite voters and making deep inroads with some traditionally Republican-leaning groups that have shifted away from Mr. Trump following his ineffective response to the coronavirus pandemic, according to a new national poll of registered voters by The New York Times and Siena College. Mr. Biden is currently ahead of Mr. Trump by 14 percentage points, garnering 50 percent of the vote compared with 36 percent for Mr. Trump. That is among the most dismal showings of Mr....