"Supports single payer healthcare" is used against Sanders in the primary to instill fear in Democratic primary voters that he'll be too far to the left for moderates and then lose to a Republican. It wouldn't be effective, or attempted, if he was already the President.
They could easily have used the same style of attacks that were used against Trump. "Kids in cages" was the case under Obama, and Trump, and Biden, but we only heard about it under Trump.
They wouldn't have used the same issues because those are the issues you use against a Republican, but the same format works against anyone. Under Sanders there would have been huge concerns about the deficit. Any tax increase impacting the middle class would be condemned but the middle class would inherently have to pay more to have single payer healthcare, so you fight it with "tax increase bad" not "healthcare bad." Not having to pay health insurance premiums would either be ignored or condemned as a giveaway to employers.
They'd find some black families who support school vouchers to go on TV and accuse him of being a racist for supporting existing "racist public schools" and things along those lines.
The general idea is to be horrified to learn that things that have been happening for years have suddenly been discovered happening under the target administration, but choose the things related to the direction they want to move. As if the Overton window is not only not covering what they want to change, it no longer even covers the status quo, and we have to go in the opposite direction or he's literally Hitler (or, presumably, Stalin). So you burn up all their political capital just to stand still while turning the viewer against them no matter what they do. Then four years later the status quo remains intact.