WASHINGTON — Senator Clinton’s plan to rescue homeowners with a $30 billion federal aid package on top of a 90-day ban on foreclosures and a five-year freeze on interest rates for subprime mortgages could actually make market conditions worse, critics say.

Private companies may be less likely to lend if they know the government can come in and override the deal, some conservative scholars warn, or they may add costly premiums to account for the uncertainty.

Mrs. Clinton added new measures yesterday to a proposal that would significantly increase the government’s role in trying to prop up the housing market and stave off further waves of foreclosures across the country.

She called for the federal government to back private sector auctions that would allow at-risk homeowners to restructure their mortgages and said if that doesn’t work, the government should “stand ready” to step in and purchase, restructure, and resell the mortgages itself.

The former first lady is also proposing a housing stimulus package to the tune of $30 billion that she said would help states and localities acquire foreclosed properties, provide aid to homeowners, and secure neighborhoods in danger of blight and crime because of widespread foreclosures and vacancies.

“If the Fed can extend $30 billion to help Bear Stearns address their financial crisis, the federal government can provide at least that much in assistance to help families and communities address theirs,” Mrs. Clinton said in what her campaign billed as a major policy speech at the University of Pennsylvania.

She called on President Bush to appoint a high-level “emergency working group” to examine how to broadly restructure faulty mortgages and keep people in their homes. She pointedly cited two former chairmen of the Federal Reserve, Paul Volcker and Alan Greenspan, along with a former Treasury secretary, Robert Rubin, as respected economists who could form a nonpartisan commission representing a diversity of views. As a Citigroup executive, Mr. Rubin has overseen billions of dollars in losses stemming partly from the subprime crisis.

Mrs. Clinton has tried to seize on the housing and credit crisis to demonstrate her leadership on the economy and appeal to the lower- and middle-income Americans who have been most directly affected. Yesterday was at least the third time she has added to her proposal since late in the summer.

At the core of her proposal are two ideas that have drawn the most criticism: a 90-day moratorium on foreclosures and a five-year halt on resets of adjustable rate mortgages for owner-occupied homes.

“The main effect will be to drive up mortgage costs in the future,” a senior fellow at the libertarian Cato Institute, Daniel Mitchell, said of the rate freeze proposal. Lenders, he said, would charge more because of the uncertainty created by the government’s intervention.

Mrs. Clinton’s Democratic opponent, Senator Obama, also criticized the proposal last month in Texas. “It will reward people who made this problem worse. It will also reward people who are wealthy and don’t need it,” he said, according to the Chicago Tribune. “A blanket freeze, as she has proposed, will drive rates through the roof on people who are trying to get new mortgages to buy or refinance a home.”

The Republican National Committee also derided the plan. “Critics agree that Senator Clinton’s housing plan could be disastrous for already strapped homeowners nationwide,” a spokeswoman, Amber Wilkerson, said. “As she has demonstrated time and again, Senator Clinton’s big government solutions cannot be trusted to alleviate the economic burdens facing hard-working American families.”

Responding to criticism that a rate freeze would force the government to override private contracts, the Clinton campaign says the New York senator is only saying that as president, she would call on major stakeholders to voluntarily agree to such an arrangement. An adviser said Mrs. Clinton had not proposed a detailed set of limits for what mortgages would be eligible for a rate freeze, other than to prohibit speculators or owners who do not live in the homes from taking advantage.

An Obama adviser said yesterday that the Illinois senator was supportive of a voluntary rate freeze for a targeted set of mortgage holders, which is already part of the Bush administration’s Hope Now plan, announced in December.

A senior fellow at the Heritage Foundation, David John, said a mandatory rate freeze would be “unfeasible” and would open up the government to liability for losses suffered by mortgage servicers. It also puts homeowners who secure a fixed-rate mortgage at a disadvantage, he said. “None of this is fair to someone who has bought a house and scrimped and saved during hard economic times to pay a mortgage,” Mr. John said.

The Clinton campaign’s economic policy director, Brian Deese, said her proposal was not a bailout for borrowers. “It’s not a free lunch,” he said in an interview. “They still have to pay their mortgages every month, often at a high teaser rate.”

Mrs. Clinton’s proposal for a $30 billion aid package and a government role in guaranteeing mortgage auctions is a more aggressive and expensive version of a plan put forward by Senator Dodd of Connecticut and Rep. Barney Frank of Massachusetts.

She characterized the steps as appropriate at a time when, she said, there is “a crisis of confidence in our country.”

“Some may claim that the plan I offered today is a bailout,” Mrs. Clinton added. “They’ll argue that it’s not the government role to help. Well that is the same type of tired rhetoric we’ve been hearing for years now, and I think the American people know better.”