For years, the F.H.A. operated largely out of the public view. But it has become a subject of controversy recently even as it has ballooned in size. Some of the agency’s critics want it to tamp down risk by insuring fewer loans; others think it should help the market by insuring even more.

As of December, the F.H.A. was insuring 5.8 million single-family residences that had a total loan balance of $750 billion. More than half a million of the loans were seriously delinquent and heading toward foreclosure.

Many of these troubled loans were made in 2007 and 2008 as the market was plunging. Last fall, the agency said its cash reserves had tumbled to 0.5 percent of its loans outstanding, far below the 2 percent mandated by Congress.

Left largely untouched by the changes is the most controversial aspect of the agency’s program: a provision allowing buyers to make a down payment as low as 3.5 percent. Private lenders these days require at least 15 percent.

Borrowers who want to put the minimum down will now be required to have credit scores of at least 580, a relatively poor figure. Previously, there was no minimum score. But this rule might have little effect. The agency says that in practice, new borrowers already have much higher scores.

F.H.A. critics argue that the agency is allowing people to become homeowners while requiring relatively little of them, which they see as a replay of the poor lending standards that created the housing boom and subsequent decline.

Agency officials have responded by saying that they have adequate safeguards in place to make sure that borrowers are creditworthy, and that these loans are saving the housing market from collapse.

Lou Barnes, a loan officer with Premier Mortgage Group in Colorado who is among those who think the government is not doing enough to support the housing market, said the changes were not unduly restrictive. He noted that the insurance premium was merely returning to its level of a decade ago.

“The F.H.A. has done its best to protect the taxpayer, and the least harm to the credit supply,” Mr. Barnes said.

An industry consultant, Howard Glaser, said that with “the F.H.A. hovering around 40 percent of new loan originations, even small rule changes echo.”

Mr. Glaser, a former official with the Department of Housing and Urban Development, which includes the FHA., said that “obtaining credit will be a little more expensive or it may be a little more difficult to qualify” but that the changes were “not enough to have a systemic impact on slowing home buying.”