That's funny, because Bernie has been beating Trump in head-to-head polls ever since 2016. And yeah, everybody knows he's a socialist. The problem is, Republicans have been crying wolf about socialism for so long -- calling Obama a socialist, for example -- that they've essentially made that word meaningless.
Think of the top ten or fifteen most successful nations, including the USA. With the exception of China, every single one of them is a mixed-market social democracy -- including the USA. The most visible difference between us and them (say, US vs. EU) is that our government is about 25~30% of our economy, whereas their governments are about 35~50% of their economies. If we were to enact the most sweeping and consequential aspect of Sanders's policy proposals, Medicare For All, it would put us at the low end of that 35~50% scale.
At the same time, despite a 4% increase in FICA (excluding the first $29k of income), with all premiums, copays, and deductibles gone, it would leave several thousand dollars more in the pockets of the average family each year. So it would be a great relief to vast masses of people.
To visualize this change, consider that the Affordable Care Act ("Obamacare") constrained health insurance companies by requiring that they spend at least 80% of revenue on actual health care instead of overhead (administration, advertising, profits). By comparison, Medicare's overhead costs are more like 3~5%, which means we can save roughly 15% overall on health care. So if health care currently costs $3 trillion per year (it's probably much more) and we can save 15%, that's $450B -- every year! Talk about "economic stimulus"... imagine what that much cash in the hands of consumers would do for the GDP.