Prior to 1983, social security was not taxable. In 1983, Joe Biden voted in favor of taxing 50% of social security – and it passed. In 1993, Joe Biden doubled down and was the deciding vote in raising the percentage taxed on social security from 50% to 85%.
The higher taxes from the 1993 change are directed to the Medicare trust.
One could see all this as a negative and indeed some people will use it against Biden in 2020.
In fact those changes (or other changes) were needed then to keep Social Security paying full benefits.
Nearly thirty years later we are worse shape and still Congress has not acted to fix Social Security finances. Instead, every proposed fix or change is categorized as cutting benefits, used for political purposes and assures nothing happens.