There are 4.2 million Democratic voters in Pennsylvania, about 325,000 more than last year.

Of the new voters, the Obama campaign believes it has registered 200,000 supporters. That's in sync with polling of new voters, indicating 62% of new voters support Obama.

If they turn out, that's a potential sleeper vote that at minimum will keep the race close.

How many Pennsylvanians will actually vote?

So far this year, most primary states have garnered turnout rates in the high 20s or mid-30s. Only New Hampshire has broken 50%. California and Ohio broke 40%. Massachusetts and Vermont just missed with 39%.

Let's presume an above average turnout of 40%. That would mean 1,680,000 voters.

Now, I'm not terribly confident that new voters are being picked up in the polling of "likely voters." Most recent polls are showing Clinton ahead between 5 to 10 points.

Let's set aside 325,000 new voters out of that estimated 1,680,000 turnout, leaving us 1,355,000. And let's give Clinton a 10-point advantage with those voters.

That would give Clinton 745,250 votes, and Obama 609,750.

Now let's add in the new voters: 200,000 for Obama, 125,000 for Clinton.

That would give Clinton 870,250 voters, and Obama 809,750, or Clinton 52%, Obama 48%.

The new voters -- presuming they turn out in full -- slash Clinton's lead from 10 points to 4 points.

Food for thought. We'll find out tonight.