A long-held stake by a handful of hedge funds may finally yield returns under the Trump administration, but it risks sending shockwaves through America’s $12 trillion mortgage market.

Last month, President Donald Trump said he had plans to take mortgage financing giants Fannie Mae and Freddie Mac public. Such a move would end 17 years of federal government conservatorship over the two companies, which have played a central role in America’s housing finance system by providing liquidity to the mortgage market.

Some experts warn that severing Fannie and Freddie from government control could raise mortgage rates and restrict access to popular mortgage products — like the 30-year fixed loan — at a time when housing affordability remains out of reach for many Americans. Last week, Senate Democrats sent a letter to William Pulte, who leads the Federal Housing Finance Agency, asking him to pause efforts to take the two public, citing the risk that it could increase costs for homebuyers.

A group of investors has been anxiously awaiting the day Fannie and Freddie return to the public markets. None has been more vocal than billionaire investor Bill Ackman, whose hedge fund, Pershing Square Capital, is one of the largest holders of common shares in Fannie and Freddie.