They will never, ever charge an ex-president or first lady with a serious crime and have him or her stand trial. It is too much of a threat to the office of the presidency and the USA's status as the only current world superpower, leader of the free world and the big stick we carry around the world. Imagine what Putin, North Korea, China, Syria, etc. would say.
The U.S. can only exert its might and hold itself out as a role model if the people at the top are not criminals. The presidency serves as the country figurehead much like the queen does for the U.K.
James Comey acquiesced to this policy only to protect the nation's standing and authority worldwide now and in the future, but he didn't like doing it. He clearly went out of his way to trash Hillary to make it clear he does not approve of her behavior.
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This unwritten policy is why Nixon was pardoned. Reagan got away with Iran-Contra. Bush got away with lying about WMD to start the Iraq war and blowing Valerie Plame's cover, and the Clintons got away with every major scandal and investigation. Robert Ray said Hillary was dishonest during Filegate and Travelgate investigations, but no charges were filed due to lack of sufficient evidence to mount a successful prosecution. Isn't that exactly what Comey said yesterday?
Bill Clinton's impeachment was not deemed a serious threat to the office. It came out of a civil lawsuit – but charging an ex-first lady who is about to be nominated for president with serious felonies such as espionage is way too dangerous.
The Clintons know this, and they have leveraged this de facto immunity to the hilt since 1993.