Fears about a looming fiscal crisis at the end of the year are starting to pinch job growth and threatening to undercut the nation’s fragile recovery, a growing number of economists and employers say.

Federal Reserve Chairman Ben S. Bernanke, in testimony Thursday before Congress, repeatedly warned about the so-called fiscal cliff — a reference to the expiration of tax cuts Dec. 31 and the imposition of automatic spending reductions Jan. 1.

By some accounts, the U.S. economy could see an unprecedented fiscal hit of as much as $720 billion if the slated changes take effect.