Nearly five years after the government took over the mortgage giants Fannie Mae and Freddie Mac, Congress is slouching toward remaking how Americans buy homes.

Gingerly, Sens. Bob Corker, Republican of Tennessee, and Mark R. Warner, Democrat of Virginia, have been working up a bill. Yet it's striking how much the process is being dominated by emotional battles and financial interests.

In the right corner — politically as well as figuratively — we have the contingent that despises Fannie Mae and Freddie Mac. These people continue, against the evidence, to consider them the central cause of the financial crisis. Their preferred solution is to wipe these companies from the earth and somehow get the government out of housing. The hope is that a thousand flowers will bloom on their graves, as private investors rush in to finance mortgages.

In the left corner — politically speaking, we are talking left of center — is a group of financiers that favors a plan to bring back Fannie and Freddie. The argument, forwarded by the banker James E. Millstein, who served as the chief restructuring officer in President Obama's Treasury Department, is that they can be fixed.

Investors like the hedge fund manager John Paulson and Bruce Berkowitz of Fairholme Capital Management have embraced this idea, contending that Fannie and Freddie can pay back taxpayers, be recapitalized and live again. Not incidentally, they could profit handsomely if this works, as they have bought up positions in Fannie and Freddie. It's a time-honored strategy: Make one Wall Street investment and then make a second investment in some Washington lobbying to protect the first.

The Corker-Warner plan creates a government insurance operation, similar to the Federal Deposit Insurance Corporation, that would insure mortgage-backed securities. Private investors would have to shoulder the first losses, probably about 10 percent. Taxpayers would not have to bail out those investors should things go south.

It's an appealing notion and the plan has commendable aspects. But if the system worked as advertised, it could make the next housing crisis worse.

To understand why, we should revisit what we have learned about the American housing and mortgage market.

First, we learned that the housing market is so central to people's wealth and the economy that the government will try to save it in a crash. At least Corker and Warner's plan grasps that, unlike the most fervent conservatives.

Second, Fannie and Freddie were fatally flawed. They were hybrids, privately held institutions with government charters — along with too much political influence and too little capital. Investors believed they were implicitly guaranteed by the government, and so they were. (Shareholders got hugely diluted, but not wiped out.) The plan tries to solve this by making the insurance explicit and then supposedly cutting off the private players from the government trough.

Neither of the two senators' offices made someone available for comment. A statement from Corker's spokesman emphasized the plan's protection for taxpayers; Warner's added a goal to maintain access to credit.

The Corker-Warner proposal, which borrows ideas from the recent Bipartisan Policy Center proposal and the left-leaning Center for American Progress, depends on getting three things exactly right. Private investors will need to have enough incentive to buy the securities (or, to use the jargon, there will need to be adequate liquidity in the market). These private entities will also need to put up enough money to have enough skin in the game to prevent taxpayers from taking a bath on the mortgages. On top of that, these firms and insurance companies will need enough capital to prevent taxpayers from having to step in to take over the companies.

There's reason to be skeptical that Congress will succeed in fine-tuning all of this. Unless the new insurance corporation regulates all housing-related investors, they will be subjected to different oversight from different agencies. Typically, businesses will gravitate to the most lenient agency and the one requiring the least capital.

If the Corker-Warner proposal were to go through, the private companies that have pole position would be the private mortgage insurers. The Republican Party has a fondness for this industry, going so far as to blow it a kiss in the party's 2012 platform. Uncomfortably, private mortgage insurers were quietly a major part of the problem after the housing bubble burst. They were woefully undercapitalized and have been operating almost as zombie institutions.

The 10 percent private investor number also poses a concern. It's a satisfyingly high number. But it's a number that will probably create big problems when housing goes into a downturn. Remember the Great National Housing Crash? Private investors fled. Fannie and Freddie needed to step in to provide liquidity. And the real Big Daddy was the Federal Reserve.

Without some mechanism to ease the requirement, the contemplated reform could exacerbate a panic, not ameliorate one. A Harvard finance professor, David S. Scharfstein, has a proposal that would remedy this, with the government stepping in during a crisis, ramping up its mortgage insurance business only in a downturn when private investors are fleeing.

What's striking is how much we've learned about housing since the crisis that isn't reflected in the reform efforts.

We have learned, for example, that the mortgage servicers have been unholy disasters, foreclosing on homeowners incorrectly, fighting principal reduction and dragging their feet on mortgage modifications that would have helped people stay in their homes. One lesson, then, is that separating mortgage servicing from ownership is a bad idea. The banks that kept loans on their books have been more ready to work out loans to keep people in their homes. The current Washington plans don't do much about this.

We've also learned that having an oligopoly of giant banks controlling the mortgage market leads to higher rates. And, because of the backlash against Fannie and Freddie, we are in danger of turning against the idea that the government has an important role in providing access to credit for those who might not be able to otherwise buy or rent homes.

Corker and Warner nod to providing greater access for small banks to compete with the big boys. And it provides a mechanism to provide access to housing for the credit-impaired. But in their fixation with solving Fannie and Freddie, the current Washington efforts give these important reforms short shrift.

The work is in the early stages. But the narrowness of the conversation is troubling.