"I think Bill Richardson has the life experience and work experience to be a truly great president," Mr. McIntosh said. "America could use a great president about now."
He said he will start working for Mr. Richardson on March 15.
Mr. McIntosh has been the state party's director for much of the past two years.
His departure comes as South Carolina's Democrats prepare for what is billed as the first Southern presidential debate April 26 and, a couple of days later, elect a new chairman at their state convention.
Mr. McIntosh was hired to lead the party's day-to-day operations in March 2005. He took an extended leave last June to run Robert Barber's unsuccessful campaign for lieutenant governor.
Mr. Richardson has been tied up getting his education agenda through his Legislature's short session this year, campaigning in Nevada - the nation's second caucus - and raising money in New York, California and New Mexico, lobbyist Crawford Cook said.
Mr. McIntosh will be replaced at the state party helm by Morton Brilliant, who helped run Democratic Gov. Jim Hodges' 1998 campaign.