That was the message delivered today to New York Mayor Michael Bloomberg by consumer activist Ralph Nader, who warned Mr. Bloomberg, "If some egomaniac is going to jump in and screw up this election, it's going to be me."

Mr. Nader established an exploratory committee for a presidential bid today to let Mr. Bloomberg know that there was "only room for one self-absorbed gas-bag in the 2008 race."

At a press conference in Washington, Mr. Nader said that voters who...