Souters retirement would give Obama an opportunity to put his stamp on the nine-member court. Seven of its members were appointed by Republican presidents, including Chief Justice John Roberts, who was nominated by George W. Bush.It will also prove to be an early, and perhaps major, test of party-switcher Sen. Arlen Specters (Pa.) fealty to the Democratic Party, as well as the new Republican leadership of the Senate Judiciary Committee, which will be responsible for vetting Obamas pick. Specter had served as the GOPs top Republican on the Judiciary panel, a post he gave up when he bolted to the Democratic ranks earlier this week. The most likely successors are Sens. Jeff Sessions (R-Ala.) and Orrin Hatch (Utah), both conservatives who have taken a tough stand on judicial nominees in the past. Keith Koffler contributed to this report.