Souter and Breyer turned out to be more left than anticipated. I think it’s a given that Kagan will be more left than she portrayed herself to be or than Republican senators who voted for her convinced themselves she is. She may be a leftist replacing a leftist, but I’m convinced she is more left than the justice she is replacing. I hope this qualitative difference may not harm the country in a material way.
But it comes down to Supreme Court votes. You have four auto on the left, four auto on the right, with Kennedy swinging between the two. Kagan would be fairly to the left of Stevens, but will vote no differently than Stevens would have.