The intellectual force behind President Bush's plan to overhaul Social Security, the man the president calls his favorite "Democrat economist," is not an economist. He is Robert C. Pozen, a lawyer and mutual fund executive who serves as chairman of MFS Investment Management in Boston. A registered Democrat, Mr. Pozen donated money to the presidential campaign of Senator John Kerry last year and voted for him on Nov. 2. He was a classmate of Hillary Rodham Clinton at Yale Law School. But all that has not stopped President Bush from embracing Mr. Pozen's main idea to bring the nation's public...