The race for the Democratic presidential nomination is tightening, with Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE closing in on Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE in two important contests.

Sanders is leading Clinton in Missouri and has cut into the Democratic front-runner’s lead in Ohio and Illinois, according to a new survey the left-leaning Public Policy Polling (PPP).

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All three states now look like toss-ups, with the PPP poll showing Sanders trailing Clinton by just 5 points, 46 percent to 41 percent, in Ohio.

The change is particularly notable because several polls in Ohio at the beginning of the month showed Clinton with a 20- to 30-point lead over the Vermont senator.

The race has drawn much closer since Sanders's surprise win in Michigan; he also trailed by double-digits in that state weeks before its primary.

In Illinois, Clinton gets 48 percent in the PPP survey compared to 45 percent for Sanders.

In Missouri, Sanders gets 47 percent to 46 percent for Clinton, according to PPP.

Clinton has larger leads in two other states holding contests on Tuesday: Florida and North Carolina.

Florida is the biggest prize of the night and will dole out 246 delegates. The former secretary of State leads in the PPP poll 57 percent to 32 percent.

In North Carolina, she gets 56 percent compared to 37 percent for Sanders.

Clinton has dominated Sanders in the South, sweeping every state by big margins. Black voters have given Clinton a cushion, rallying to the former first lady's side in huge numbers.

However, Sanders won a higher proportion than expected of the black vote in Michigan, which suggested his anti-trade, anti-Wall Street rhetoric may be winning support from a coalition of voters.

Other polling released Monday from Quinnipiac University found nearly identical results in Florida and Ohio, with Clinton leading by 26 points in Florida, 60 percent to 34 percent, and by 5 points in Ohio, 51 percent to 46 percent.

Tuesday is a critical day in the race for the Democratic nomination.

Clinton has a huge lead over Sanders in total delegates. According to The Associated Press, she holds 1,231 delegates compared to 576 for Sanders. Those totals include superdelegates, the lawmakers and other party officials who have direct votes at the end of party’s nominating contest. Clinton leads Sanders with these voters, 465 to 25.

If Sanders wins Ohio and takes three of the five states holding contests on Tuesday, it would be another shot of momentum for his insurgent campaign.

It’s possible Clinton could still win more delegates with big victories in Florida and North Carolina but close losses in the other three states. Still, the losses would add to the sense that she's limping to the finish line.

For Clinton, resounding victories in Ohio and Florida would do much to quiet talk that she is a flawed candidate.

A loss in Ohio in particular will raise new questions about whether Clinton can hold the White House for Democrats in the fall, given that state’s importance in the general election.