As Pennsylvania voters head to the polls today, they'll invariably cast their ballots not only as citizens of the Keystone State but as representatives of their various demographic groups: the working-class, old, educated, suburban, late-deciders. (They're probably not saying, "Well, I'm 72, so I'd better vote for Clinton," but the political pundits and superdelegates still have them pegged.) Some groups are simply more important than others (sorry, "drug users," "gymnasts"), and they are the ones that could decide this primary, and possibly the entire race.
• Suzanne Smalley writes that the affluent Philadelphia suburbs will be a crucial battleground, and Obama could surprise everyone if he does better than expected here. [Newsweek]
• Gary Langer thinks the single most important demographic factor is education, and Obama pulls in the majority of college-educated voters. But watch out for the late-deciders, who have been turning to Clinton in recent primaries. [Numbers/ABC News]
• Katherine Q. Seelye looks at the importance of age. Throughout the campaign there has been a sharp divide between young voters, who overwhelmingly vote for Obama, and older voters, who generally support Clinton. Pennsylvania has the second-highest proportion of people over 65 years old in the country, and a majority of registered Democrats are over 45. [NYT]
• Noam Scheiber has a very interesting take on why the closed primary might actually help Obama. Assuming that support among Independents and Republicans is split between Clinton and Obama (as it was in Ohio) but that Obama's support is more intense, Scheiber concludes that far more Obama-supporting Independents and Republicans will have actually gone through the trouble of registering as Democrats so they could vote today. [Stump/New Republic]
• Peter Wallsten writes that some superdelegates will likely pay especially close attention to white working-class voters to see if they've turned away from Obama after Bittergate and Reverend Wright. [LAT]
• Carrie Budoff Brown and Kenneth P. Vogel focus on late-deciders, who usually end up supporting the candidate who "represents something new," but in this primary season have been moving toward Clinton. Another group to watch is college students, who some pollsters worry may not have been fully accounted for in recent surveys. [Politico]
• Ana Marie Cox writes that if Clinton wins either late-deciders, first-time voters, or blue-collar voters, she'll at least have an excuse "not to go away." [Swampland/Time] —Dan Amira
For a complete and regularly updated guide to presidential candidates Hillary Clinton, Barack Obama, and John McCain — from First Love to Most Embarrassing Gaffe — read the 2008 Electopedia.