While there is a legitimate debate to be had about whether the electoral college is good or not, it is not new, and the Democrats knew about it before the 2016 election (we hope). They surely should have noticed it after the 2000 election. In 2009 they held all the levers of power, and could have chosen to eliminate it via constitutional amendment, but did not. Regardless of your opinion on whether they should have or not, they knew that the electoral college existed, that the Rust Belt would be important, and they chose their candidate and election strategy with full knowledge. It's also the same system that elected Obama, twice, and also Bill Clinton.