Debate to highlight vanishing diversity in Democratic field

A half dozen White House hopefuls will gather on a debate stage in Des Moines, Iowa, on Tuesday night at a critical moment in the Democratic primary.For a Democratic field that was once the most diverse in history, the debate stage will be the starkest example yet of the challenges facing candidates of color.Andrew Yang, an Asian American entrepreneur who participated in last month's debate, didn't meet the thresholds for Tuesday's forum.