For Senator Barack Obama, so far success has depended on energizing new voters and collecting the support of independents.
This has manifested itself in high turn-out for both caucuses and primaries.
Some polls suggest that the Reverend Wright affair is cutting into that support.
How we will know (and what the Clintons will be looking for) is by how high the turn-out goes in the PA primary.
If the turn-out is above average, even by a few percentage points, then Obama has weathered the storm.
If the turn-out is average or below average, then Obama has been significantly hurt by the incident, and you can look for Hillary to take it all the way to the convention.