Decision day in Pennsylvania

Pennsylvania is the biggest remaining state on the Democratic primary calendar, with 158 pledged delegates at stake. The demographics of the state tilt in Clinton's favor. The median age is older, and the population is more blue collar and white than other states.

Obama hopes to do well in Philadelphia, with a large black population, and the surrounding suburbs, with well-educated and upper-income voters. He also has the backing of Senator Bob Casey, while Clinton counts Governor Ed Rendell and Philadelphia Mayor Michael Nutter among her supporters. Clinton has also been endorsed by the mayor of Pittsburgh.

"If Mrs. Clinton loses tonight, her candidacy is almost certainly finished," writes Adam Nagourney of the New York Times. "She may say she is going on when she takes the stage in Philadelphia after the polls close, but the reality is she is effectively out of money and a loss would not exactly send her already shaken donors heading for their checkbooks."

The race is very important for Hillary Clinton. "Clinton badly needs to make up ground in the delegate fight and, given the way they're distributed, that could be difficult," writes Dan Balz of the Washington Post.

The polls close at 8 p.m., but political observers offer some helpful guidance for what to watch throughout the day. Politico.com lists five signs to look for in analyzing the race. 

1. Check for turnout at 1 p.m.
2. Don't be fooled by early results.
3. Follow the undecideds.
4. Watch certain towns and neighborhoods.
5. Keep an eye on key counties.