WASHINGTON, July 19 - With his nomination of Judge John G. Roberts, President Bush moved Tuesday to plant the conservative imprint on the Supreme Court that has been a central aim of his presidency, but with a member of the Washington legal establishment designed to frustrate any Democratic effort to block Mr. Bush's replacement for Justice Sandra Day O'Connor. President Bush announced the names of candidates for federal judgeships in May 2001. Included was John G. Roberts. The president's choices reflected his desire to leave a conservative imprint on the courts. As a judge for just two years on the...