The problem is that even though the general consensus was that Roberts was equally qualified to those other justices, he was demonized and the current President and Vice President voted against him for being to far "outside the mainstream" of legal thought. The same happened with Alito. Now, I'm not arguing that turnabout is fair play but if President Obama sends another nominee as liberal as Roberts is conservative, there should rightly be some fireworks.
As Althouse writes:
And, of course, conservatives are always up — or should always be up — for a debate about how their approach to constitutional interpretation is properly and neutrally judicial and it's only the the liberal's approach that is political. That's not quite true, but the general public is immensely receptive, and the liberals know it. That's why, when their nominee comes before the Senate Judiciary Committee, regardless of the reason why she was picked — e.g., her empathy with the poor and the unfortunate — she is not going to open up and defend liberal constitutional jurisprudence. She is going to do her best imitation of John Roberts.
And that's why Bill Kristol is crushingly right: "A big debate on the Constitution, a serious debate" will benefit Republicans.