Sanders would clean the floor with her. No way the Democratic electorate makes the same mistake twice now that Trump's election has made them painfully aware of Clinton's many, many flaws as a candidate (her history of corporate flip-flopping and the fact that she has all the charisma of an intelligent trout being chief among them).
The Democratic establishment would certainly rather lose with Hillary than win with Bernie, but you're right, the electorate might not be so interested in giving Trump a 2nd term.