"[This order came from] District Court Judge Emmet Sullivan. He's an African American, he was appointed by Bill Clinton, and he is tough as nails," Klein said Wednesday to J.D. Hayworth on "Newsmax Prime."
"He once said to Hillary or Hillary's lawyers that we wouldn't be in this fix with this email scandal if an appointed official, meaning Hillary … had followed the rules and she didn't follow the rules. He's very tough.
"I think this is a major, major turning point in this whole case that he has said OK, go ahead and depose Huma Abedin, Cheryl Mills, Jake Sullivan, and all of Hillary's assistants under oath. [A] major, major turning point."
Sullivan's ruling on Tuesday means State Department officials and aides to Clinton, now the Democratic presidential front-runner, will be grilled about whether Clinton's private email system during her time as secretary of state was an effort to sidestep open records laws.
Klein said it is still possible Vice President Joe Biden could step in to run for the White House if Clinton's email scandal begins dragging her presidential campaign down. That's not something that would upset the Obama administration, he added.
"Hillary has been shoved down Barack Obama's throat. He doesn't like her, he doesn't want her. He'd much prefer Biden," Klein told Hayworth.
"He's going to let this thing take its course because if it does, there's a good likelihood that Hillary will be out and Biden will be in."
Klein is the author of "Unlikeable: The Problem with Hillary," published by Regnery.