The Federal Reserve, faced with more criticism than its leaders anticipated, stepped up its counteroffensive on Wednesday as leading Republican lawmakers continued to attack its plan to spur the recovery. The Fed chairman, Ben S. Bernanke, met with 11 members of the Senate banking committee to explain the decision to inject $600 billion into the banking system, a resumption of the Fed’s bond-buying program aimed at lowering long-term interest rates. In a speech on Wednesday, Eric S. Rosengren, president of the Federal Reserve Bank of Boston and one of the biggest advocates of the Fed’s decision, said the plan could reduce the unemployment rate by a little less than half a percentage point by the end of 2012. “This would translate into more than 700,000 additional jobs that we would not have had in the absence of this monetary policy action,” he said.