The Pa. Expectations Game

Sen. Barack Obama greets customers at Pamela's diner in the Strip District April 22, 2008 in Pittsburgh. (Getty Images)

By Jon Cohen
It is zero hour in Pennsylvania, and another opportunity to test the mettle of pollsters taking last minute reads of Democratic primary voters.

The polls become particularly important at this late date in the primary cycle, as so much of the post-election analysis will hinge on how the candidates perform tonight relative to expectations -- expectations that come largely from such pre-election polls, as well as results of previous exit polls.

And what do we (think we) know?

Publicly released polls of varying methodologies put the Democratic race at anywhere from Clinton +13 to Obama +3. Limiting the scope to traditional telephone polls with live interviewers, Clinton's late lead appears to be in the five to seven point range.

One wild card, of course, is the undecided voter, who as a group made up as many as one in eight likely voters in the LA Times-Bloomberg poll released last week. In four out the last five primaries, including Texas and Ohio, late deciders broke for Clinton by wide margins, and an analysis of those who were undecided in Pennsylvania three weeks ago showed them tilting toward Clinton (as did those who has expressed a candidate preference). But much has changed in the final weeks and days of previous contests, and Obama is heavily outspending Clinton in the Keystone state .

The Ohio primary created another bundle of expectations for the Clinton campaign. In many ways, the demographics of the Ohio Democratic electorate mirror those of the Pennsylvania voters likely to cast ballots today, and some of the voting patterns in this year's Democratic contests have fallen neatly along demographic lines. (For a detailed look at the demographic prediction game, check out Gary Langer's blog post from yesterday.) Clinton won the popular vote in Ohio by 10 percentage points.

Taken together, the "priors," as pollsters call them, point to a Clinton win by five to 10 points. But hang on for the spin from both sides, regardless of tonight's outcome.