At least Perez is being honest about their concerns over Bernie. If you remember last time, the DNC got caught providing Hillary with backdoor support against Bernie. Thus why his supporters shunned Hillary when she got the nomination. The thing is, the DNC isn't just worried about Sanders getting the nomination, they're worried about the negative impact down ticket if this happens. They would lose the House and lose even more seats in the Senate because more people would see that the Democratic Party had officially become the Democratic Socialist Party. An identity that would be almost impossible to shake. Putting forth a moderate candidate to run against Trump is crucial to save their image.