CHICAGO  James A. Johnson, the consummate Washington insider whom Senator Barack Obama tapped to head his vice-presidential search effort, resigned abruptly on Wednesday to try to silence a growing furor over his business activities.

Mr. Johnson’s departure deprives Mr. Obama of decades of experience and access to Washington’s power elite. Mr. Johnson has been a fixture in Washington political and legal circles for three decades, and he led the vice-presidential search team for Senator John Kerry, the Democrats’ presidential nominee in 2004.

His resignation, at the start of a general election contest in which the candidates have pledged to run issue-based campaigns, came after days of intense scrutiny from the news media and attacks from Senator John McCain and Republican Party officials over mortgages Mr. Johnson, a former chief executive of Fannie Mae, received on favorable terms from the Countrywide Financial Corporation, the mortgage company that was a central player in the subprime lending crisis. Mr. Johnson also faced questions about his role on compensation committees that awarded large payouts to corporate executives.

His resignation highlights the difficulties for Mr. Obama’s campaign in trying to live up to his promises to remain independent of the Washington establishment and the special interests that populate it.