NEW YORK—Coronavirus deaths are rising in nearly two-thirds of American states as a winter surge pushes the overall toll toward 400,000 amid warnings that a new, highly contagious variant is taking hold. As Americans observed a national holiday Monday, New York Gov. Andrew Cuomo pleaded with federal authorities to curtail travel from countries where new variants are spreading. Referring to new versions detected in Britain, South Africa, and Brazil, Cuomo said, “Stop those people from coming here … Why are you allowing people to fly into this country and then it’s too late?” The U.S. government has already curbed travel from some of the places where the new variants are spreading—such as Britain and Brazil—and recently it announced that it would require proof of a negative COVID-19 test for anyone flying into the country. But the new variant seen in Britain is already spreading in the United States, and the …