Republicans were well aware of Souter's judicial philosophy when they confirmed him to the Court. Here are some quotes from Souter during his confirmation hearing.
"I believe that the due process clause of the 14th amendment does recognize and does protect an unenumerated right of privacy." p. 54
"So, it seems to me that the starting point for anyone who reads the Constitution seriously is that there is a concept of limited governmental power which is not simply to be identified with the enumeration of those specific rights or specifically defined rights that were later embodied in the bill." p. 55
"I do not believe that the appropriate criterion of constitutional meaning is this sense of specific intent, that you may never apply a provision to any subject except the subject specifically intended by the people who adopted it." p. 129
United States Senate Committee on the Judiciary, S. Hrg. 101-1263, Nomination of Stephen G. Breyer to be an Associate Justice of the Supreme Court of the United States
These are just some of the quotes that Souter made to express his judical philosophy, there are plenty more. He was clearly not a justice in line with "original intent." The Republicans in the Senate at the time knew it and they confirmed him anyway on a vote of 90-9.