How should Hillary respond to the Bernie challenge?

[publicunofficial]

Kill him with kindness. Reframe his big issues with your own proposals. Never condemn him.

This. Assauge progressives with some lofty proposals, probably continue her trend of stealing Elizabeth Warren talking points, and Bernie's support will either fade or cease to grow.

The worst thing she could do is go onto the debate stage and call him a crazy communist.

[publicunofficial]

Is Sander's age an issue? Reagan was 69 in 1980 and 74 in 1984 and lived to be 93.
Admittedly it could be an issue, but it didn't hurt Reagan, not much. Moreover, many people thought that Reagan was too conservative to be electable. Likewise arguing that Sanders is too liberal may end up being a mistake. I am not aware of any polls that show how he would do in the general election. Are there any?

Reagan's age may have well been a problem; there's a very good chance that Reagan began suffering from dementia towards the end of his presidency, and was diagnosed with Alzheimer's not long after leaving office.