Sanders calls himself a "Democratic Socialist" and he explains that in detail to anyone who asks. Labels are irrelevant...

"Credible candidates"? What do you want? Ms. Clinton has credentials to spare... and a command of the issues rivaling Bill Clinton! Do I automatically with anything she says or does? No... but I didn't totally agree with anyone from FDR on!
Which British politicians do you find totally credible? A Democracy is, by definition, subject to some odd choices at all levels... witness the current Republican circus! Sanders accepts Democracy, but appends certain aspects of 'socialism' to it... meaning fair treatment for **everyone** and changes in tax systems and voting procedures to attain this. He has no vision of anything like Communism.

Ake... you presume a lot from your perch 'over there'. You cannot possibly discern the nuances of either this election or the American system in general. You DO echo some of the sour grapes remarks some of US make by just waving their arms and branding all current politicians as "bought and biased and useless". There are 500+ members of Congress and many thousands of lower level people in this country... some of them are fools, and some of them are brilliant. It is work to sort them out in a Democracy.
I can name a dozen that I would find acceptable to see the highest offices... and there are more that never get noticed; but when the media spotlight hits in an important election, the steam roller of mud-slinging begins trying to spin ANY data into accusations of incompetence and lying and such.... sadly, it is SO much work to sort the wheat from the chaff that many voters merely take the easy path and follow 'talking points' and slogans.
You, sir, simply fall into that trap.