By Marilyn Lewis of MSN Real Estate

After more than 40 years of subsidizing and boosting homeownership, the federal government is talking about backing away. The Obama administration wants to eliminate federal guarantees for home loans for all but creditworthy buyers "with modest incomes" who otherwise could not get a mortgage from a private lender, according to a report that the administration gave to Congress in February (PDF).

Change like that could make buying a mortgage more expensive. Americans' favorite home loan, the 30-year, fixed-rate mortgage,  would lose ground against other mortgage types.

There's even talk that the popular 30-year loan could become extinct, though that's unlikely.

"There would definitely be fewer 30-year mortgages, but they would not disappear," says Dean Baker, co-director of the Center for Economic and Policy Research in Washington, D.C. He wrote the books "Taking Economics Seriously" and "False Profits: Recovering from the Bubble Economy."

It's all talk, at this point, about how to shrink, change or eliminate Fannie Mae and Freddie Mac, the two huge, government-run corporations that have kept costs low for middle-class homeowners by guaranteeing home loans.

Massive defaults by homeowners, along with management and accounting scandals at Fannie and Freddie, are costing taxpayers hundreds of billions of dollars. Even political rivals agree it's time for a new approach.

The debate among regulators, economists, politicians, consumer advocates and lobbyists could continue for years before Congress passes a plan, experts say. After that, any changes would be phased in slowly over many more years.

Read:  Why Fannie and Freddie may never die

Meanwhile, homeowners may wonder how this change could affect mortgages today and in the long term.

What's happening to 30-year mortgages?
Today, 80% of all mortgages are 30-year, fixed-rate, "conventional" loans, Freddie Mac says. "Conventional" means Fannie and Freddie can guarantee them, as long as they're below a maximum amount, so they're cheaper. By spreading lower payments over decades, conventional loans more expensive in the long run, but they've allowed many people to buy a home.

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