The tightening race could lead to some fireworks at tonight's debate.
Clinton is trying to woo young women. She is also trying to win voters' trust.
She could be on the attack tonight, especially if health care comes up. She has criticized Sanders support for a single payer health plan.
Sanders is expected to keep touting his plans to tackle economic and social inequality and systemic racism. He still trails Clinton in the upcoming states, but he is on the upswing.
Tonight's debate is in Wisconsin, where Republicans control the state government in the swing state.
They have cracked down on collective bargaining in recent years.