I wonder who felt like attacking Sanders from the left would be a good idea...
Attacking Sanders for wanting to impose regressive taxes on the poor and middle class is not attacking him from the left.
You keep repeating this allegation, but have yet to substantiate it, as far as I have seen. Link please.
He is citing Bernie Sanders' own filed legislation to establish a single payer system, which would replace medicaid and private insurance with a single payer system funded by a 6.7% increase in the payroll tax(regressive) and a modest 2.2% increase in the income tax, with higher taxes for those making more.
It is laid out very clearly in this
article in the
Washington Post.