Ralph Nader is a political activist, author, lecturer and attorney.
At the Springfield Republican Party meeting, Mr. Burns asked the members what act of unmitigated evil should they do. Nader picked up his hand, but Mr. Burns told him that he has done enough. This is a reference to how, in 2000, Ralph Nader, running on the Green Party ticket as a response to an increasingly corporatist Democratic Party, won enough votes in swing states such as Florida to deny Democrat Al Gore (who otherwise may have won a majority of those votes) the Presidency.
During the song "High to Be Loathed", Mr. Burns mentioned Nader and made his shadow, among with many other villains.