Recent polling averages in the states that are voting on Tuesday -- Florida, Illinois, Missouri, North Carolina and Ohio -- shows that Clinton leads in the four where there have been recent polls. She leads by a bit in Ohio and her home state of Illinois and by a lot in Florida and North Carolina. (The only recent poll in Missouri gives Bernie Sanders a 1-point lead.)
But here's the thing: Whether or not Clinton wins Ohio doesn't really matter.
It's important to remember that the Democrats, unlike the Republicans, don't allocate delegates on a winner-take-all basis. When Donald Trump won South Carolina with a plurality of the vote, he got all of the state's 50 delegates, a total that right now constitutes more than half of his lead. There are no states like that on the Democratic side. There are some variations in how the states divvy up their delegates, but they're proportionally distributed from now until the primary is over.
Which is why the 2008 daily delegate totals looked like this.
As Clinton tried to play catch-up with Barack Obama, he would get some delegates every time she did. The only times she made big gains against him was in states she won by a wide margin. But the proportional delegate system kept Obama steadily out of reach.
It's worth comparing Obama's 2008 lead in the delegates to Clinton's. Clinton, by virtue of huge margins of victory in Mississippi, Alabama and Louisiana, has a much bigger lead than Obama did at this point -- or than Obama did at any point. (The data below excludes superdelegates.)
Sanders has won states by big margins, too -- Kansas and Vermont -- but they have far fewer delegates to award.
Let's say that Clinton and Sanders tie in Ohio, Missouri and Illinois and she wins by a 20 points in Florida and North Carolina. Per some back-of-the-envelope math, Clinton would get about 380 delegates to Sanders's 315 -- increasing her lead by about 60 delegates. Even if Sanders wins Illinois, Missouri and Ohio, Clinton will still net more delegates if she wins Florida and North Carolina big.
Cook Political Report's Dave Wasserman did the math over the long term:
If delegates split evenly on Tuesday, Sanders needs more than 55 percent of all of the rest of the delegates to tie Clinton.
It's also worth noting that Sanders has been strongly helped by the presence of independent voters in the Democratic contests so far. In Michigan, for example, nearly a fifth of the vote was independents voting for Sanders (he won 71 percent among the 27 percent of the electorate that was independent) according to exit polls -- more than enough to have swung the race to Clinton had the primary not been open to independents.
Even with his support from independents, Sanders is at risk of continuing to fall further behind in the delegate count. Which is exactly why the two parties' nomination contests look very different over the long term, even though they're close right now: Clinton's lead is much less vulnerable than Donald Trump's.