HEMPSTEAD, N.Y. — In a combative opening debate, Hillary Clinton emphatically denounced Donald Trump Monday night for keeping his personal tax returns and business dealings secret from voters and peddling a “racist lie” about President Barack Obama. Businessman Trump repeatedly cast Clinton as a “typical politician” as he sought to capitalize on Americans’ frustration with Washington. Locked in an exceedingly close White House race, the presidential rivals tangled for 90-minutes over their vastly different visions for the nation’s future.