McCaskill spokeswoman Adrianne Marsh e-mailed this morning to refute the claim, writing:
I believe the website www.porkbusters.org only utilizes select votes to present skewed information that frankly misrepresents the records of the newer members of the Senate. The tally actually counts five of the 12 votes they base their ranking system on against Senator McCaskill when she was actually not even in the Senate to cast those votes. Additionally, the database doesn't even take into account whether or not the Senator actually participates in the earmark process, which Senator McCaskill has abstained from this year.
The study did, in fact, count against her record five votes that took place before McCaskill took office. In the voting opportunities that transpired on her watch, she was 2 for 7 in voting for reform. (According to porkbusters' accounting method, that would give the senator a 28.5 -- good for 42nd-best, in the middle of the pork pack.)
She has been praised in the past for her attempts to declassify defense appropriations.
McCaskill points primarily to her support of the DeMint amendment in January, which was intended to "disclose the size, purpose and sponsor of earmarks slipped into congressional bills." She came under some scrutiny for this too, however, and in January was named "Porker of the Month" by a group called Citizens Against Government Waste for her initial vote against the amendment. After some language in the act was changed, it unanimously passed the Senate.
You can come to your own conclusions about McCaskill's record on pork-barrel spending by looking at her voting history.