Washington ? The Obama administration wants to shrink the government’s role in the mortgage system — a proposal that would remake decades of federal policy aimed at getting Americans to buy homes and would probably make home loans more expensive across the board.

The Treasury Department rolled out a plan Friday to slowly dissolve Fannie Mae and Freddie Mac, the government-sponsored programs that bought up mortgages to encourage more lending and required bailouts during the 2008 financial crisis.

Exactly how far the government’s role in mortgages would be reduced was left to Congress to decide, but all three options the administration presented would create a housing finance system that relies far more on private money.

“It’s clear the administration wants the private sector to take a more prominent role in the mortgage rates, and in order for that to happen, mortgage rates have to go up,” said Thomas Lawler, a housing economist in Virginia.

Abolishing Fannie and Freddie would rewrite 70 years of federal housing policy, from Fannie’s creation as part of the New Deal to President George W. Bush’s drive for an “ownership society” in the 2000s. It would transform how homes are bought and redefine who can afford them.

Treasury Secretary Timothy Geithner said the plan would probably not happen for at least five years and would proceed “very carefully.” In the meantime, he said the companies would have the cash they need to meet their existing obligations.

“We think there’s very broad consensus on the Hill and in the broader private market that there needs to be a transition to a much smaller role for the government,” he said.

Ever since the housing market went bust and the country fell into a financial crisis, pressure has been building for the government to do away with Fannie and Freddie and reduce taxpayer exposure to risk.

Fannie and Freddie own or guarantee about half of all mortgages in the United States. Along with other federal agencies, they played some part in almost 90 percent of new mortgages over the past year.