Political Insider

Nearly every poll shows Sen. Hillary Clinton on her way to a victory over Sen. Barack Obama in the Pennsylvania primary. We won't fall into the trap of poll averaging, but will instead list the most recent polls here:

• Zogby: Clinton +10
• SurveyUSA: Clinton +6
• Suffolk: Clinton +10
• Quinnipiac: Clinton +7
• Strategic Vision: Clinton +7
• American Research Group: Clinton +13
• Mason-Dixon: Clinton +5
  
• Public Policy Polling: Obama +3
  

As in all elections, the results tonight will depend on turnout. In fact, the difference in the polls above are almost entirely on how they predict turnout.

Should Clinton win Pennsylvania by five points or less, it will widely be interpreted as an indication her voters stayed home. If she wins by double-digits, it will be due to her supporters flocking to the polls. To reasonably continue her campaign of convincing superdelegates about her electability, Clinton needs a big victory.

Meanwhile, as Ben Smith notes, Obama couldn't have had a worse few weeks. He's been on the defensive, had his worst debate performance of the campaign and seems more interested in running out the clock on Clinton than inspiring his own supporters. His attacks on Clinton over the last few days are an obvious attempt to get Clinton voters to stay home.

So after 15 months of campaigning, the Democratic presidential nomination fight comes down to Clinton's margin of victory in Pennsylvania. A 10+ point lead will be seen as a big Clinton win while anything less than five points will be a big Obama win. However, if Clinton's victory is between five and 10 points, the campaign continues.