Doesn't seem much different than Sanders supporters or Sanders himself getting branded socialists or commies, or Trump supporters getting branded nazis or racist bigots or Trump being compared to Hitler for running the nationalist populist platform and saying politically incorrect things. I'd say it's just the "us vs them" mentality at work.
Trump gets weird labels because he is weird and he proclaims his own weirdness. Normal republicans dont get that. Normal democrats do. When Rubio says that Obama wants to make American not be great, it's hum-drum politics as usual. It's only when he screws up and repeats himself three times that people think it's absurd. What happens constantly and is considered not worth commenting on is often quite important but hard to even notice. We expect democrats to prove they dont have very, very unusual views.
There is a tiny, tiny fringe in the left that wants America to not be great. The best place to find them it to go through the comments section of foreign policy online magazine articles where they will be insisting that Russia or Iran or China needs to kick America in the teeth to save the world. Obama is expected to prove that he is not a member of this radical fringe that wants America to fail. Rubio on the other hand is not expected to prove that he doesn't support kicking all muslims out of the country. It would be weird and offensive to suggest he did. There is no left wing equivalent attack on the right to what Rubio can say about Obama with no one batting an eye. And Obama is the president, the dude in the middle of politics.
We label things partisan politics when they are exclusively attacks levied leftward. And not just by the far right, it's considered perfectly centrist to bemoan Obama as a paternalistic socialist. But with Sanders/Clinton we see punching rightward in a way that doesn't normally happen. If I said that all Sanders supporters were to blame for stealing data or sexist bullying or belittling black people, it would justifiably be dismissed as absurd. But when the Sanders supporters (the man himself doesn't do this) make the accusation towards the right, it's considered just partisan politics. And it's an unusual thing, Obama supporters didn't accuse Clinton of being on the take because she supported the individual mandate back in 2008.
The essential thing is that we are labeling behavior partisan that exists entirely in one direction.