The characters state that there is no liberal on the court. In the first season Bartlett put Justice Mendoza on the bench, who was quite liberal. If for some reason Mendoza had left the bench, Bartlett would have appointed another liberal, so no matter what there would have been a democrat on the bench.
Final scene in the W.H. press room when President Bartlett is at the podium introducing his two new Supreme Court nominations, he incorrectly introduces Glenn Close's character, Judge Evelyn Baker Lang, as the next Chief Justice of the Supreme Court. While she will be the Chief Justice of the Supreme Court, the correct title should be "Chief Justice of the United States."