State Department officials are delighted by Mrs. Clinton’s new role. Since the president’s planned trip to South America this year was scrapped, sending the First Lady is the next best thing. And they love the impact back home. “She makes a great case for foreign aid,” says one official. White House aides say Mrs. Clinton is surprised and thrilled by her success overseas. “It’s a great thing for Hillary,” says an aide. “She’s really found her voice here.”