NEW YORK (CNN) - It's certain to be long night, but here are some early indicators I'll be keeping my eye on as the first wave of exit polls roll into the CNN Election Center.
The gender gap:
The bigger it is, the more it helps Hillary Clinton. She needs to do well with women voters tonight in order to carry Ohio and Texas. In nine Super Tuesday primary states Clinton won, men were split, but nearly 60 percent of women voters supported the New York senator. In seven states Obama won that night, the Illinois senator beat Clinton among both men and women.
Much attention has been focused on Obama's ability to rally young voters. The number of these voters that showed up to vote and the proportion that break for Obama will be key to his chances of winning tonight. Senior voters have always been a backbone of support for Clinton, and will likely continue to be so today. In many of the states Obama has won, the youth vote has been heavy and has broken for him better than 2 to 1.
Minority turnout will be crucial, but which minority? African-American voters have solidly aligned themselves behind Obama, and Latinos have usually supported Clinton. This means Obama has had the edge in states where African-American voters make up a significant proportion of the electorate while Clinton has had the advantage in states where Latinos do so. But Texas has both a heavy African-American population and a heavy Latino population. Whichever group has a higher turnout could be key to who wins the state.
Whichever candidate wins among union voters in Ohio could very well carry the state. Even though several major unions in Ohio - like the Teamsters and the Service Workers and the Food and Commercial Workers - have endorsed Obama, the Nevada caucuses proved that union voters don't necessarily vote the way their leaders tell them to. Clinton is banking on strong support from this demographic to help her win tonight.
How much of a role will they play? Independent voters are allowed to vote in all four states today. In the past, these voters have strongly favored Obama. Will their turnout be high enough to tip the scales toward the Illinois senator tonight?
Republicans are allowed to vote in the Democratic primary in Texas. Some conservative commentators, like Rush Limbaugh, are urging them to vote for Hillary Clinton just to keep the Democratic race going. The idea that large numbers of Republicans will vote for Clinton just to make trouble for the Democrats seems unlikely. But lots of unlikely things have happened this year.
Watch Bill Schneider break down the demographics of the March 4 primaries
Related: CNN's Ed Lavandera reports on early Texas voting
–CNN Senior Political Analyst Bill Schneider