It has been said that a large turnout benefits Democrats. That is true. It is true because there are more Democrats (50%) among registered voters than Republicans (42%).
So, how is it that Republicans have done so well in the last couple of off-year elections (2010 and 2014)? They did well because their voters went to the polls and too many Democrats didn't bother to vote.
Democrats have a golden opportunity to flip the House of Representatives (and maybe even the Senate) this year. But that will only happen if they can get their voters to the polls. It does no good for Democrats to outnumber Republicans if Democrats stay home on election day.
Republicans do best with members of the Silent generation, rural voters, Whites, and men.
Democrats do best with urban voters, Millennials, Generation X, College grads, Post-grads, Asians, Hispanics, Blacks, and women.
The split between the parties is very close among suburbanites, Baby Boomers, and those with less than a college degree,
These numbers are from seven surveys done by the Pew Research Center in 2017. They surveyed 10,245 people nationwide, and the margin of error is 1.1 points.