Singer Beyoncé is the latest superstar entertainer to endorse Kamala Harris's bid to become the next US president.At a rally in the city of Houston, Texas, she told the crowd she wasn't there as a celebrity or a politician, but as a "mother who cares deeply about the world my children and all of our children live in, a world where we have the freedom to control our bodies."She went on to urge Americans to "sing a new song".Donald Trump has also been campaigning in the traditionally Republican state.