Clarke says the two former Bush administration officials parted with Clarke telling McClellan, "OK. Good luck. Be careful."
Clarke says he left McClellan alone, a position the former press secretary is finding whereever he turns.
McClellan's brother Mark, who was a member of Bush's Council of Economic Advisers and later appointed by Bush to head the FDA, issued the following statement, "Scott has thought long and hard about his experience in Washington, and his book reflects that. I have the deepest respect for my brother's commitment to public service, and for his saying what he feels needs to be said to help our government work effectively."
ABC News' Mark Mooney contributed to this report.
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