I'm liberal but I approve of his method. This is about upholding the law. I think it is a mistake to think that liberals want to stack the courts. We simply believe in the righteousness of our stance.
For example, in the case of Dredd Scott, it was not only agaisnt the constitution to consider him still a slave, it was not moral. So, in the end of the day, as long as conservatives try to pull at the fabric of society to make it conform to their religous and at times, white supremacist ideolgoy, they will continue to be viewed as people who do not care about the law.
Liberals believe in equal rights and justice. Pick a case. Consider fairness and you will find the liberal perspective in the case. The courts do not have the right to dictate law. The legislative branch is responsible for creating laws. The executive (the president) enforces laws and the judicial branch is supposed to act like a referee. I agree with Roberts that judges should not try to make laws from the bench.