So far in 2010, Citigroup shares are still up 12% but they were up more than 50% in 2010 at one point.
Ackman gained fame for profiting handsomely from shorting bond insurer MBIA (MBI - Get Report) and for taking a big equity stake in shopping mall owner General Growth Properties (GGP - Get Report) for pennies on the dollar after it had filed for bankruptcy protection. The shares are now worth more than $13.
Ackman's positive stance is in line with recent bullish comments from Rochdale Securities analyst Richard Bove, who characterized the stock as "seriously undervalued" earlier this month, and Oppenheimer analysts who boosted their rating on the shares to outperform on May 26.
-- Written by Dan Freed in New York.