Majority PAC, a Democratic super-PAC, is buying ad time to help out Sen. Bob Casey (D-Pa.), the latest sign that his race against businessman Tom Smith (R) has tightened.

Smith, a coal mine owner, has been able to dominate the airwaves with millions of dollars of his own money, and recent polls show him pulling to within single digits of Casey. Some Democrats have grumbled that Casey has been slow out of the gate with his response, though he's been spending more heavily in recent days.

The group has bought a half-million dollars in air time in the Pittsburgh market, where Smith is from and where voters have been moving away from Democrats up and down the ticket in recent years, according to a Republican ad tracker.

Casey is still the clear favorite in this race, but that Smith has made it a contest at all is notable.