The unsettled race has seen surges at various points by Biden, Warren, Sanders and Buttigieg, though it's become defined by that cluster of shifting leaders, with others struggling for momentum. California Sen. Kamala Harris, once seen as among the top tier of candidates, shelved her campaign this month, citing a lack of money. And Warren has become more aggressive, especially toward Buttigieg, as she tries to recover from shifting explanations of how she’d pay for “Medicare for All” without raising taxes.