It has been said that the Sanders supporters helped elect Donald Trump by staying at home or voting for a third party candidate. But it is worse than that. Around 12% of his supporters actually voted for Trump in the election -- a candidate that supported none of Sanders' progressive agenda. And that was enough to put Trump over the top in Wisconsin, Michigan, and Pennsylvania (states that no one expected Trump to win, and without the Sanders supporters, he would not have won).
Here is part of an article about this by Jason Le Miere in Newsweek:
Bernie Sanders supporters switched their allegiance to Donald Trump in large enough numbers last November to sway the election for the real estate billionaire, according to an analysis of voter data released Tuesday by the blog Political Wire. Since Trump’s shock victory over Hillary Clinton, much discussion has focused on the degree to which passionate Sanders supporters’ refusal to embrace Clinton led to the Republican winding up in the White House.
According to the analysis of the 2016 Cooperative Congressional Election Survey, fewer than 80 percent of those who voted for Sanders, an independent, in the Democratic primary did the same for Clinton when she faced off against Trump a few months later. What’s more, 12 percent of those who backed Sanders actually cast a vote for Trump.
The impact of those votes was significant. In each of the three states that ultimately swung the election for Trump—Michigan, Wisconsin and Pennsylvania—Trump’s margin of victory over Clinton was smaller than the number of Sanders voters who gave him their vote.
While the self-confessed socialist Sanders and the “America first” embracing Trump represent polar opposites on the ideological spectrum, they both carried an outsider message and appealed to voters frustrated with the staid political establishment. Although Sanders railed against the policies and rhetoric offered up by Trump, the Republican at times sought to reach out to Sanders voters, emphasizing the similarities of the two candidates’ positions on issues like trade.
The data would suggest that Trump’s efforts were at least in part successful.