If one thinks the Nazis were actually Socialists,
this might be a good read.
Sanders is a Social Democrat, which is perfectly okay. The systematic imbalances in the system we have may make it impossible for him to enact any policies however, which would be an indictment of the system.
There are three major problems I see today with American democracy: money, unproportional representation (through gerrymandering or just geography), and efforts to make it harder for political opponents to vote. Sanders will not be getting much funding from large donors, particularly not in the form of a Super PAC. To win control of the House the way districts are currently drawn, Democrats would most likely need to win by more than 6% nationally: most likely, just barely. The Supreme Court's removal of sections 4b and thus effectively 5 of the Voting Rights Act has allowed many southern states and municipalities to pursue policies designed to make it harder for democratic leaning demographics to vote such as certain voter ID laws, the reduction of early voting, and other measures.
That said, the effect of funding on a campaign can easily be exaggerated, with gerrymandering wave elections actually return a disproportionately massive shift, and at least in the short term efforts to restrict voting which are not as effective as the old Jim Crow laws tend to be fairly effective at getting out the vote they're seeking to suppress.
The greater problem unfortunately lies with Congress, which has gradually become more and more dysfunctional. This is tied to two problems: increasing polarization of the voters, tied perhaps to polarized news and civic society; and relentless focus on fundraising. We need to change how representatives are elected, how campaigns are funded, or both.
Unfortunately, the days of bipartisan campaign finance reform and the like are long gone. The only way I see such changes happening in the future is through a wave election with addressing these problems as the winning party's core platform.
Would any of the candidates lead to this? I doubt it.