Of all the temporary patches the U.S. government slapped onto the sinking financial system in September 2008 -- from pumping money into banks to rescuing insurer American International Group Inc. -- none was more urgent to then-Treasury Secretary Henry Paulson than saving mortgage giants Fannie Mae and Freddie Mac.

'It was the single most important thing we did to prevent disaster -- real disaster,' said Mr. Paulson in a recent interview. But five years later, he adds, 'it didn't occur to me that we'd be here with nothing done.'

Fannie and Freddie remain the largest single piece of unfinished business from the financial crisis. As record profits have replaced huge losses, some now question whether cosmetic changes could substitute for the more radical overhaul of the companies envisioned five years ago.

The profits are even giving Fannie, the bigger of the two, some of its old swagger. In an online video touting the company's profit streak, Fannie boasts that it will 'change the way people experience housing for generations to come.' Fannie had around 7,200 employees at the end of February, up from around 5,800 when it was bailed out.

The Obama administration, congressional leaders and the financial industry all insist that Fannie and Freddie must change. But figuring out exactly how to reshape or replace them is proving harder than most anyone imagined.