“It’s a case of identity theft,” Mr. Koch said of the call in which the liberal blogger got through to Gov. Scott Walker of Wisconsin, drew him out about his plans to weaken unions and posted a recording of the call on the Internet, making news and embarrassing the governor. Mr. Koch, whose company, Koch Industries, had given major campaign support to Governor Walker, among other conservative candidates and causes, added, “I didn’t even know his name before this brouhaha erupted.”

Mr. Koch joked that the call could cause him problems. “I was thinking to myself, ‘My God, if I called up a senator or a congressman to discuss something with them, and they heard ‘David Koch is on the line,’ they’d immediately say, ‘That’s that fraud again — tell him to get lost!’ ” he said with a laugh

Mr. Koch said that only a relatively small portion of his giving goes to politics and public policy — most, he said, goes to cancer research, followed by cultural and educational institutions.