There might be several reasons why Bernie is unelectable but against Trump, only one stands out as potentially fatal to a general election bid: Healthcare
Recall Chuck Schumer’s publically stated regret for taking on the healthcare issue and passing the ACA when the Democrats did. Schumer felt they should have focused all energy on the economic recovery before taking on the ACA. Whatever you think about Chuck Schumer, the guy is a master politician and knows a winning political issue from a loser. Setting aside the moral and policy imperatives for now, creating an opening for Republicans to criticize Democrats for prioritizing ACA ahead of the economy made for a high risk, low reward political issue for Democrats. Why high risk, low reward? The ACA directly impacted about 10% of voters - the uninsured and individual healthcare insurance subscribers. The 90% is all other voters covered under employer plans and Medicare, and Medicaid. Additionally, Republicans argued that 100% of voters are impacted by the economy, and the Democrats should have waited on the ACA until the economy was in full recovery.
The ACA was designed to address only 10% of the population, providing affordable healthcare to the uninsured and the individual market, and did so quite effectively. However, the ACA still remains underwater around 40/55/5 (for/against/don’t know).
Bernie’s proposal of Medicare for All will directly impact the 90%, the vast majority of whom are satisfied with their employer provided insurance and current Medicare. This is a classic the devil you know (for most, the devil they like?) vs the devil you don’t issue. From the ACA experience, one must recognize the fear power behind this issue --- 100 million voters who have good employer provided insurance do not want to move their children, family, loved ones, friends, neighbors, etc. to the unknown.
Imagine the effect of $500 million in TV and radio ads running 24/7 for a month in 9 swing states scaring the living daylights out of Republicans, Independents and Democrats as well --Democrats are not immune to fear-based political issues.
I’m not going to address the economic impacts of Bernie’s Medicare for All pledge, but it’s important to note that private healthcare is still 20% of US GDP, and they won’t go down without a fight.
Finally, the ACA was a limited plan to address a limited problem – the uninsured and underinsured. Bernie’s Medicare for All will disrupt the entire system and plays right into the hands of the Republicans.