Rubin joins Morgan Stanley's(MS) John Mack and Goldman Sachs' Lloyd Blankfein. According to the Center for Responsive Politics, Clinton leads all of the candidates in raising money from Wall Street's securities and investment firms, to the tune of $4.75 million dollars through the end of the third quarter.

The New York Times and the Associated Press report that Rubin will hold a Dec. 13 fundraiser for Clinton. The dinner represents the last major event for the fourth quarter and will be open to supporters who sell more than $25,000 in tickets.

The endorsement was no sure thing. Clinton and Rubin had a rocky relationship during the 1990s and squabbled over monetary policy. Rubin's son, Jamie, is a notable supporter of Sen. Barack Obama (D., Ill.).

Rubin is one of the big Democrats who had remained on the sidelines, and his endorsement should give Clinton a boost. The other big figure yet to endorse would be former Vice President Al Gore. In 2004, Gore endorsed Howard Dean not long before the Iowa caucus.