DETROIT — As the nation celebrates the holiday honoring the Rev. Martin Luther King, some worry that the country is not fulfilling the dream of the African American leader who strove against hate and preached racial and social justice. Nicholas Thomas is one of more than 100 volunteers who will commemorate King by making improvements to a Detroit neighborhood. He says he is worried that the United States is suffering from deep racial divisions under President Donald Trump. For many people of color, the political climate heading into the 2020 presidential election includes signs of exclusion, with the front-runners all white men and women.