This notion that liberal democratic positions are somehow radical is a huge red-herring promoted by the rightwing and reported by a lazy and compliant media.

Most of the general positions proposed by democratic candidates are supported by large pluralities and majorities:

In June Gallup reported:

• 47% of Americans say they would vote for a socialist. (Not great as far as electoral prospects to be sure, but certainly significant enough to be considered solidly mainstream.)
• 74% of Americans believe corporations wield too much influence in American life and politics. Only 37% believe unions do so.
• According to Pew, 58% of Americans would support breaking up big banks.
• 64% support regulating carbon emissions.
• 79% want the wealthy to pay more taxes. 53% of Republicans want people making $1M to pay more.
• 92% of Americans want a society with less income disparity than currently exists. More than 90% of Americans preferred Sweden's income disparity model to that of the US. *69% of Americans believe the government should help reduce the gap between the rich and everyone else. *54% don't believe that political donations should be considered protected free speech.
• 80% support requiring employers to provide paid maternity leave. 85% support a paid sick leave requirement.
• Most Americans support the right of workers to unionize and large majorities support the rights of teachers and other public workers to unionize.
• 50% want medicare for all socialized healthcare. 71% support a public option.
• 60% support same sex marriage.

Its pretty clear that the positions articulated by some of the democratic candidates may be new or novel to some but they are far from being extreme.