The Federal Reserve took a dramatic step Wednesday to help revive the economy, resuming its unconventional efforts at stimulating growth nearly a year after embarking on an initiative that ultimately failed to deliver a healthy recovery.

The Fed’s latest move aims to lower interest rates on mortgages and other long-term loans without making another major infusion of money into the economy — and brushes aside a crescendo of criticism from Republicans who have been making the Fed a campaign issue.

The announcement that the Fed would buy $400 billion in long-term Treasury bonds immediately achieved its intended effect, pushing rates on these securities and other investments to their lowest level in decades.

But the stock market rendered a sharply negative verdict. The Standard & Poor’s 500-stock index tumbled almost 3 percent on the Fed’s discouraging statement that its leaders see “significant downside risks” for the economy. Asian markets closed down between 2 and 4.85 percent, and key European indexes were trading more than 4 percent lower at midday.

“They succeeded at getting lower long-term interest rates, but the confidence they wanted to encourage didn’t seem to come through,” said Jim O’Sullivan, chief economist at MF Global. “That has to be disappointing.”

In deciding to put their foot back on the pedal, Fed officials rebuffed calls from Republican congressional leaders, in a letter sent Monday to Fed Chairman Ben S. Bernanke, that the central bank refrain from taking new steps to spur growth, fearing they could actually harm the economy. There was also opposition inside the Fed’s policymaking board, with three members dissenting from the decision.

The Fed action, which capped a two-day meeting, is focused squarely on lowering mortgage rates in an effort to strengthen the ailing housing market and lighten the load of the tremendous debt weighing on consumers. The move could also make it cheaper for businesses to borrow money for investments and push more dollars into the stock market.

The program of buying long-term Treasury bonds will continue through June. Instead of paying for the bonds with newly created money, as the Fed did during a $600 billion program that ended three months ago, the central bank will use proceeds from selling off $400 billion in shorter-term securities on its books.

The bond-buying program that ended in the summer, though massive in scale, failed to keep economic growth from sputtering. The disappointing result showed the limits of what the Fed can accomplish at a time when consumers are struggling with enormous debts and the U.S. banking system remains traumatized. The new initiative could face the same constraints.

The Fed’s strategy is often called a “twist” operation because it simultaneously pushes long-term rates downward and short-term rates upward. And that is exactly what happened in financial markets after the announcement. The interest rate on 30-year Treasury bonds fell two-tenths of a percentage point to drop below 3 percent for the first time since records were kept.

In a separate step also aimed at lowering mortgage rates, the Fed announced it will begin re­investing the proceeds of mortgage-related securities it owns into the same types of securities. By increasing the demand for these investments, the Fed should make it cheaper for mortgage lenders to get the money they use to make home loans.

The actions taken by the central bank are unlikely to guarantee a stronger economic recovery, but they could lower rates for 30-year fixed-rate mortgages by two-tenths of a percentage point, private forecasters have said.

The steps come with some risks for the Fed, including the danger that it will lose money when it comes time to sell off the bonds.

The Fed took action in response to evidence that the U.S. job market is weakening — job growth came to a halt in August — and waning concerns about inflation as fuel prices have fallen. In its statement, the Federal Open Market Committee said that information received in recent weeks “indicates that economic growth remains slow” and that “recent indicators point to continuing weakness in overall labor market conditions, and the unemployment rate remains elevated.”

Responding to similar concerns about the job market, President Obama proposed a $447 billion plan this month to try to boost employment through a combination of tax cuts and new government spending. House Republicans remain resistant and are especially opposed to Obama’s proposal that taxes for the wealthy be increased to help pay for his jobs plan.

After reviewing the Fed’s statement, Peter Hooper, chief economist of Deutsche Bank Securities, noted that “the degree of concern about the possibility of dipping into recession has increased.” He pointed to language in the Fed statement about disruptions in financial markets, which reflect heightened concern about the European debt crisis threatening banks on both sides of the Atlantic.

The Fed stopped short of taking even more dramatic action that would have entailed printing new money to buy bonds. The Fed board also declined to set specific numerical targets for unemployment or inflation that would have to be reached before interest rates were raised. Several Fed officials have been advocating that approach.

Still, the Fed’s initiative is likely to run afoul of Republican critics who have been attacking Bernanke’s “easy money” policies. Four Republican Congressional leaders wrote in a letter to Bernanke this week that he should eschew any further steps aimed at stimulating the economy.

“Respectfully, we submit that the board should resist further extraordinary intervention in the U.S. economy,” wrote House Speaker John A Boehner (Ohio), Senate Minority Leader Mitch McConnell (Ky.), Senate Minority Whip Jon Kyl (Ariz.) and House Majority Leader Eric Cantor (Va.). “We have serious concerns that further intervention by the Federal Reserve could exacerbate current problems or further harm the U.S. economy.”

Bernanke and his policies were already under attack by Republican presidential candidates, who have argued that low-interest-rate policies are spurring inflation.

Even internally, there was disagreement. Dallas Fed President Richard Fisher, Minneapolis Fed President Narayana Kocherlakota and Philadelphia Fed President Charles Plosser “did not support additional policy accommodation at this time,” according to the statement from the Open Market Committee.