Santorum aired a spot featuring actors supposedly portraying four big donors to Casey's campaign meeting in a smoke-filled jail cell. The senator's campaign later conceded that none of the men had given money to Casey's Senate campaign and that two had contributed to Santorum's campaign, which donated the money to nonprofit groups.
It's probably not a good idea to run an ad that ends up highlighting your own connection to maleficent characters. I'm not sure how the ad came to be, but I certainly have a guess. Did you know that the Senate still does not file their campaign finance reports electronically? They actually still turn in paper reports. Those reports then have to be manually entered into electronic form at great cost to taxpayers, and it takes weeks to do.
The article cited above also lists a couple of polls for the Santorum / Casey match up. The Keystone poll shows Casey leading 45-38 with the Green candidate getting 5%. An IssuesPA/Pew poll showed Casey leading 47-28. Both polls were of registered voters, not likely voters which probably accounts for the difference in the level of Santorum's support. Regardless, an incumbent not even polling in the forties at this late date almost always ends up looking for a new job.