Ackman's disclosure came two days after another investment firm, Fairholme Capital Management, offered to buy Fannie and Freddie's core businesses from the government in a $52 billion deal.
Miami-based Fairholme, led by Bruce Berkowitz, made the proposal to the Federal Housing Finance Agency, which oversees Fannie and Freddie.
Fairholme said it would lead a group of investors to buy the mortgage-bond guarantee businesses of the two companies. The firm said that would be sufficient to back about $1 trillion in new mortgages.
The Obama administration is seeking to wind down Fannie and Freddie.
The goal of the Obama plan is to replace Fannie and Freddie with a system that would put the private sector, not the government, primarily at risk for the loans. That would spread the risk over numerous companies. The government would still be involved, both in oversight and as a last-resort loan guarantor. The White House also wants a guarantee that private lenders will make sure homeowners have access to 30-year fixed mortgages.
It's unclear whether the Fairholme proposal would fit with the administration's plan. Government officials have said previously that a small group of investment firms taking on such a large risk of mortgage defaults would open the possibility of the government having to bail out the firms if they failed.