DES MOINES, Iowa (Reuters) - Joe Biden is still the front-runner in the Democratic presidential race, but the crown is slipping.On a two-day holiday visit to Iowa, the state that kicks off the White House nominating fight in seven months, Biden was greeted by new polls showing his lead narrowing in the state and nationally after a wobbly performance in last week’s first debate. Biden’s halting response to a bruising debate attack on racial issues from rival Kamala Harris, a black U.S. senator from California, has some Democrats in Iowa rethinking their support for the former vice president.