I can't think of a Supreme Court justice who, starting off a tad left of center, edged rightward over the course of a few terms. The reverse is often true of course, which makes one ask why. Perhaps after book and dinner parties with the liberal Georgetown set and media elite you start to take on the coloration of your companions' politics, unless you already have a reputation as firmly conservative. It's human nature. Thus it's going to be really, really tough for Justice Kennedy to reject Obamacare. It's such a high-profile case that Kennedy's legacy with the writers of history, mostly liberal academics, would be in jeopardy. What is needed is for the dinosaur media to come down hard on Kennedy, to overplay their hand (as they did with Justice Thomas). They need to be so heavy-handed that it pisses Kennedy off. Not likely to happen, as the wooing of moderate Republicans by the media is legendary.
Almost by definition if a case has reached the Supreme Court then it's a tough call. And Supreme Court justices are humans, not judicial robots. Some justices see themselves as independent, non-ideological judges -- beware the case that threatens to dismantle that reputation. And this time around, unlike with Bush v. Gore, Kennedy doesn't have O'Connor to give him non-ideological cover. It's going to be interesting.