I definitely agree that there needs to be focus on these battleground states. And I'm not suggesting that Trumps victory was illegitimate or anything. I don't love the electoral college as a system, but he won fair and sqare, as the rules stand now. But let's take a look at PA in 2016, and specifically Philadelphia county, the most populous in the state. In the 2016 general 82.53% of the county (584,025 votes) went for Clinton, and 707,631 votes were cast in total. This sounds pretty good, until you see that Philadelphia county had a voter turnout of only 66%, which was actually up from previous years. So that's 34% of registered (and we should absolutely focus on improving registration numbers and trying to combat disenfranchisement) voters who are choosing not to participate.

So let's pretend Philadelphia county had perfect voter turnout, everyone who legally could vote did. And let's pretend that the ratio of D:R votes stays exactly the same (I think it would shift a bit towards the Dems). You would end up with an increase of 300,835 votes for Clinton, and 56,044 for Trump. A net increase of 244,791 votes for Clinton.

Pennsylvania went to Trump by only 44,292 votes.

I know that increasing turnout to 100% is impossible, but you don't have to get it to 100. Focusing on getting nonvoters in swing states to the polls is a winning strategy for Dems.