2016 isn't a tenuous situation for Democrat senators. They're all running in Blue states.
In 2014 they did constantly.
I think we’re seeing a federalization of senatorial elections, where the red states have slowly shed their Democrat senators while the blue states are still slowly rejecting their Republican senators.
The Democrats have fewer red state senators to worry about defending, so they can act more radically without having to worry about saving the skin of senators coming from more conservative states. This would be fine.. if we had a Republican caucus with enough backbone to fight this.