There are certainly people out there who think Bernie Sanders is too far left or is unelectable because he will be rejected by moderate Democrats and independent voters. I am not one of them. I think his Medicare for All and his plans for attaining fossil fuel independence in ten years are actually laudable, but utterly impractical and thus in reality will never see the light of day because they will never get past Congress. In the end, Sanders will have to sign whatever he can get from Congress and it will actually look a lot like what the candidates to his right are proposing.
I am also not worried that much about him turning off moderates and independents and thus handing Trump a win. It is of course possible but the presidential election is a numbers game. You need to get the most electoral votes, nothing more or less. So blue states are going to give all their electoral votes to whomever the Democratic candidate is. Even if having Sanders on the ticket lessened Democrat turnout or caused more independents or Republicans to come out to vote against him, it won't be enough to move the vote of blue states to Trump because it's winner takes all.
It could give Trump higher margins in red states but again like the blue states, it doesn't really matter. Sanders doesn't need the red state votes. He only needs to add those three states: Wisconsin, Pennsylvania and Michigan to what Hillary Clinton got in order to win and his appeal to the working class and the young makes it reasonable to believe he will get it. It might be close but it seems likely to me he will get it.
So why am I wary. It's not because I think his chances aren't good to win the presidency, it's because he may have a negative down ballot effect in red or purple states where we need to flip the Senate either by turning off Democrat leaning voters and/or eliciting an even stronger turnout from Republicans and independents. Without flipping the Senate, a President Sanders will not pass any legislation of significance. Most importantly he will not be able to pass legislation to shore up our failing democracy including better protection for whistle blowers and more legal consequences for White House employees who do something that breaks the law, even if the president tells them to do it. Additionally, without the Senate a President Sanders will not be appointing anyone to the Supreme Court or likely any other judges either. If Ruth Bader Ginsburg retires in the next four years, McConnell will keep her seat open until such time as a Republican president is elected. In other words, Bernie Sanders is likely to win the presidency but with his hands more or less permanently tied behind his back. As with Obama, McConnell will do his best to leave Sanders with nothing to show at the end of 4 years, a serious blow for someone who got elected promising revolutionary change.
I hope I'm wrong but I am wary and I believe others will join my wariness if he wins the nomination and is then hit with a tsunami of "existential threat" political ads on TV and social media not to mention business and industry leaders proclaiming that his election will end capitalism and with it jobs in America and the stock market swooning to new lows in anticipation. These things will scare many voters and fear is a powerful force in politics. This despite the fact as I pointed out that they actually have nothing to fear.
This is the problem with Sanders. To accomplish the bold things that people like he or Warren want to accomplish requires building up a groundswell of support across the U.S. for those things for many years so that voters will have that person's back when the opportunity comes. They have not done that. The Republicans on the other hand have done so with people like Rush Limbaugh and Sean Hannity. There's a lesson to be had but we may pay a terrible price to learn it.