The major problem seems to have been that neither party had or has an effective answer to the systemic de-employment (i.e. substituting lower paying jobs) which affects manufacturing hubs post-free trade agreements.

The economist argument has always been (a) sign free trade deal, (b) some jobs lost as labor reallocates to cheaper locations, (c) prices for goods decrease enough that everyone is better off.

Based on the result from the Rust Belt (and the split, not just the winner), I'd say they felt neither party did enough about (b).