LAS VEGAS (Reuters) - Broad-based support across age, racial and ideological groups propelled Bernie Sanders to a dominant victory in Nevada’s Democratic caucuses, tightening his grip on the front-runner spot in the race to find a challenger to President Donald Trump. Joe Biden, a former vice president, appeared headed to a badly needed second-place finish in Nevada after poor showings earlier this month in the first two nominating contests in the Democratic presidential race ahead of the Nov.