WASHINGTON — Congress approved mortgage relief for 400,000 struggling homeowners Saturday as part of an election-year housing plan that also aims to calm jittery financial markets and bolster the sagging economy. President Bush said he would sign it promptly, despite reservations.

The measure, regarded as the most significant housing legislation in decades, lets homeowners who cannot afford their payments refinance into more affordable government-backed loans rather than losing their homes.

It offers a temporary financial lifeline to troubled mortgage companies Fannie Mae and Freddie Mac — pillars of the home loan market whose losses have sparked investor fears — and tightens controls over the two government-sponsored businesses.

What began as a showdown between the White House and the Democratic-led Congress over how far the government should go in rescuing homeowners evolved into a bipartisan effort that could be the last such compromise before Bush leaves office in January.

In a rare Saturday session, the Senate voted 72-13 to send the bill to the president; the House had passed it Wednesday.

Bush had withdrawn his veto threat earlier in the week over $3.9 billion in neighborhood grants. He contended the money would benefit lenders who helped cause the mortgage meltdown, encouraging them to foreclose rather than work with borrowers.

"Because of the Democratic Congress' delays and the need for action now, President Bush will sign this bill when he receives it, despite our concerns with some provisions, including nearly $4 billion to help lenders, not the homeowners this legislation is intended to serve," said Tony Fratto, deputy White House press secretary.

Democrats won cherished priorities in the bargain: the aid for homeowners, a permanent affordable housing fund financed by Fannie Mae and Freddie Mac, and the neighborhood grants.

The legislation takes several approaches to curing the ailing housing market.

It aims to spare an estimated 400,000 debt-strapped homeowners, many of whom owe more their houses are worth, from foreclosure by allowing them to get more affordable mortgages backed by the Federal Housing Administration.

The FHA could insure $300 billion in such mortgages, which would be available to homeowners who showed they could afford a new loan. Banks would first have to agree to take a large loss on the existing loans in exchange for avoiding an often-costly foreclosure.

The plan also is designed to relieve a broader credit crunch that has taken hold because of rising defaults and falling home values. To free up safer and more affordable mortgage credit, the bill permanently would increase to $625,000 the size of home loans that Fannie Mae and Freddie Mac can buy and the FHA can insure. They also could buy and back mortgages 15 percent higher than the median home price in certain areas.

The measure tries to prevent blight in areas hardest hit by the housing crisis, where waves of foreclosures have left properties sitting abandoned, dragging down property values and ruining neighborhoods. It sends $3.9 billion to such neighborhoods to buy and fix up foreclosed properties.

It also beyond the current housing crisis by overhauling the Depression-era FHA. It requires lenders to show how high a borrower's payment could get under the terms of his mortgage and provides $180 million in pre-foreclosure counseling for struggling homeowners.