What's interesting about 1920 is that the Democrats basically got their BEST candidate and still went down in flames. Of course, back then, in the absence of television and/or debates, the qualities of the individual candidates probably didn't count for much.
Sometimes the basic circumstances decide an election; the candidate only matters how big you're winning or losing. For example in 1964; no Republican would have won against LBJ, or no Democrat would have won against Nixon in 1972. Wendell Willkie was also a great candidate (as a devoted Democrat I would have voted for him unless FDR is the Democratic nominee), but also lost by a wide margin.
I realize that, but what I'm saying is that Cox was probably their best candidate yet still lost by a '64/'72-style landslide (well in the popular vote...maybe not electoral vote). It's hard to imagine another Democrat doing any worse (Kentucky was the only close Cox state), but I can't think of who would've done better either.