Jeffrey M. Lacker, the Federal Reserve's most persistent internal critic, does not much resemble a firebrand. He is personally cheerful, professionally inclined to see both sides of an issue and quick to acknowledge he may not be right. He says he would rather be wrong.

But for the last several years, Mr. Lacker, president of the Federal Reserve Bank of Richmond, has warned repeatedly that the central bank's extraordinary efforts to stimulate growth are ineffective and inappropriate and, worst of all, that the Fed is undermining its hard-won ability to control inflation.

Last year, Mr. Lacker cast the sole dissenting vote at each of the eight meetings of the Fed's policy-making committee, only the third time in history a Fed official dissented so regularly.

"We're at the limits of our understanding of how monetary policy affects the economy," Mr. Lacker said in a recent interview in his office atop the bank's skyscraper here. "Sometimes when you test the limits you find out where the limits are by breaking through and going too far."

As the Fed enters the sixth year of its campaign to revitalize the economy, the debate between the Fed's majority and Mr. Lacker - whose views are shared by others inside the central bank, as well as some outside observers - highlights the extent to which the Fed is operating in uncharted territory, making choices that have few precedents, unclear benefits and uncertain consequences.

The economy continues to muddle along, shadowed by the threat of another government breakdown, and the crisis of high unemployment is only slowly receding. But in trying to address those problems by suppressing interest rates, the Fed risks the unleashing of speculation and inflation.