The Comey firing had nothing to do with the Hillary Clinton email investigation. It was all because Trump was outraged over Comey's public acknowledgement that the FBI was investigating his Russia ties. He wanted the investigation to disappear, and he began obsessing about firing Comey—presumably in hopes that this was all it would take to kill the case. And apparently Trump was shocked when Democrats didn't line up behind him. They hate Comey too, don't they?
The Politico story is based on conversations with insiders who were happy to confirm that the Comey firing was all about Russia. This directly contradicts the White House narrative that it was about the fact that everyone had lost confidence in Comey because of the way he mistreated poor Hillary Clinton. Who are these people who work for Trump (?) but are happy to undermine him to the press on a regular basis?