WASHINGTON  The Justice Department has shut down a wave of high-profile investigations of members of Congress over the past few months, drawing criticism that the government’s premier anticorruption agency has lost its nerve after the disastrous collapse last year of its case against former Senator Ted Stevens.

This month, lawyers for Senator John Ensign, Republican of Nevada, announced that federal prosecutors had told them they would not charge the senator with conspiring to help a former aide, with whose wife he had had an extramarital affair, violate a lobbying law.

A few days later, lawyers for Representative Jerry Lewis, Republican of California, facing scrutiny for steering government spending to campaign donors, said they were told that their client would not be charged, either.

Other federal corruption investigations known to have been ended without charges this year had focused on Tom DeLay, the former House majority leader and Republican of Texas; Representative Don Young, Republican of Alaska; and Representative Alan B. Mollohan, Democrat of West Virginia.