Sanders was ahead of Clinton in one survey of Missouri released Monday by Public Policy Polling and within a few points of the Democratic front-runner in PPP polls of Illinois and Ohio.

It’s possible Clinton could still win more delegates even if Sanders wins all three states, by keeping the margin close and winning big in Florida and North Carolina. But the loss of Ohio, in particular, would raise questions about her candidacy. It would also give Sanders first-place finishes in seven of the last 11 contests.

Clinton enters Tuesday with a lead of over 200 pledged delegates, a gap that will be very difficult for Sanders to close. Her lead jumps to more than 600 delegates when superdelegates — the party appointees who can support any candidate prior to the party convention — are included.