Cooper was with Sessions when Sessions interviewed Rosenstein. And Cooper probably approved of Rosenstein after the interview. But I believe Cooper didn't know Rosenstein before he met him at the interview.
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So I don't think it was Cooper who got Rosenstein the interview. As you say it might have been Christie.
The original story I heard was that Ron Tenpas, a pal of Chris Christie and a Big Law insider, championed Rod Rosenstein. Recall that FBI Director Christopher Wray was also Christie’s personal lawyer. So was Sessions completely fooled?
If RR is as bad as he seems, who has been in charge of the 40,000 sealed indictments? All US Attorneys report directly to RR.