Someone else already made the point that many voters are single-issue voters. If your single issue is trade or free-trade agreements and protecting American jobs, that's something that Bernie and Trump are actually close to each other on and Hillary is miles away.
There's a reason why so much union membership is peeling away from Hillary to Trump, it's because no one who's a Democrat or Republican has even pretended to care about the unions or American jobs for decades. Even if Trump is just paying lip service to protecting American jobs, that's more than either major party has done since Jimmy Carter.
Strong Unions machines won Massachusetts for Hillary where it was prime territory for Bernie to crush it.
I don't think there any real data showing Trump stealing Union votes from Democrats. They know the GOP party position and who Trump is running with.