Of course there’s a hell of a lot that can change with regards to polling between the end of a primary and a general. My point wasn’t that Sanders was up by fifteen at the end of the primary, therefore he would win by fifteen (although, wasn’t Clinton up by about two at the end of the primary, and didn’t she end up winning the popular vote by a similar margin). The point is that if you have two candidates in a primary, one who beats all of the other parties candidates in polling and beats their likely opponent by double digits, and the other one can only beat the likely opponent by less than five and loses to every other candidate, the first candidate has a far larger margin of error. If the polling was off by ten points to the final outcome (is that extreme enough a difference for you?), Sanders at the end of the primary still leads by five against Trump, but Clinton gets her ass kicked. And, as a result of polarization, the bigger question isn’t appealing to the other side as much as getting your side out to vote and getting independents (which Sanders always had an advantage with over Clinton, and likewise Trump over Clinton) and people who don’t vote (who generally favor more, not less, liberal policies (http://www.demos.org/publication/why-voting-gap-matters)).
Furthermore, if that’s all the dirt that the Republicans would have on Sanders, you’re right, it would be so damaging. I mean, that must have been why Donald Trump and Bill Clinton both lost in their Presidential bids, we would never elect someone with multiple accusations of sexual assault against them, let alone an essay from decades ago that the guy admits was idiotic. As for the commies, do you honestly think people care all that much? Hillary Clinton, and now the Democratic Party, have tried to hit Trump by accusing him of being just about being the Manchurian Candidate, and it hasn’t worked. Her campaign spent two months of the general trying to argue that Trump was in cahoots with Putin, and it did nothing other than making her campaign and foreign policy look unhinged. If that didn’t work, why exactly do you believe that accusations of Sanders being soft on Communism would work nearly thirty years after the end of the Cold War? And beyond that, if you believe that all of that would be damaging, then how do you not figure that Clinton was an even worse candidate? She not only had the email server, which always was legally dubious, and Benghazi, which did have a noticeable impact on her favorability ratings, but all of the shit from the nineties as well. You don’t to get complain that a candidate opposing you has baggage when you support someone with enough baggage to live out of a hotel for an entire year with a new pantsuit every day.
And finally, look at the data I presented. When you actually look at what people support, more liberal positions, especially on economic issues, are popular with a wide swath of the American people. I will repeat this until I am blue in the face, a majority of Americans support more, not less, government involvement in health care. They support raising the minimum wage to something higher than where it is now. And every time this gets brought up, rather than accepting that real, honest to god Democratic policies and principles, which tend to be left wing, are popular with Americans, you simply hand wave that those policies have somehow become moderate. Even in the face of Republicans screaming to high heaven about how these same policies, which are so popular, represent a communist plot, or did you not pay attention to what Glenn Beck was saying before he jumped ship from the Right? Even under your framework, what does it say that those policies have become “moderate” in the face of determined Republican opposition? What does it say other than either that more left wing policies are appealing to people when they are actually sold to people or that Americans are actually a tad more to the left on a number of key issues than both the major parties leadership is?
And when you think about it, it’s not that surprising that people will vote to improve their lives if you sell them that, not a list of Rube Goldberg policies that appeal to people who get excited at the thought of working through spreadsheets all day that no one else can understand or seriously believes will be implemented. It came out this week that Clinton was planning on making her Secretary of Labor Howard Schultz, chairman and CEO of Starbucks. A company that has had complaints of anti-Union action and had to pay $100 million in a class action lawsuit in California for wage theft. Why should anyone believe that this would be a man who cares about labor anymore than Trump’s Labor Secretary will? Clinton’s campaign couldn’t even pick whether they were in favor of a $12/hr or $15/hr minimum wage, which in addition to ignoring that labor and working class voters, in other words key Democratic constituencies, were already mobilized around the latter but furthermore fed into the perception that, to paraphrase President Obama from eight years ago, that Clinton will do anything and say anything to get elected.
As you said, the data speaks for itself, and is far more important than your Hillbot feelings. Fact of the matter is the New Democrat experiment has failed spectacularly for decades now. We either can continue to try being Republican-lite on key issues to win these mythical moderate votes, not even try to understand what it is that moderates actually support, and end up losing to actual Republicans who can get their people out, or we can try being Democrats for a change and actually stand for working and middle class voters. I’ve lead you to the water, if you don’t want to drink because it would upset your view of politics as seen on “The West Wing”, then I can’t really help you too much.