Is Turning Out the Base or Appealing to Moderates more Important?

The latest political science on how to win American elections:

Political observers, campaign experts, and academics alike argue bitterly over whether it is more important for a party to capture ideologically moderate swing voters or to encourage turnout among hardcore partisans. We speak to this debate by examining the link between the ideology of congressional candidates and the turnout of their parties’ bases in U.S. House races, 2006–2012. Combining a regression discontinuity design in close primary races with survey and administrative data on individual voter turnout, we find that extremist nominees suffer electorally, largely because they decrease their party’s share of turnout in the general election, skewing the electorate towards their opponent’s party. Along with shedding light on questions concerning the interplay of parties, voters, and candidates, the results help address the debate over swing voters and turning out the base. For our sample of elections, turnout appears to be the dominant force in determining election outcomes, but it advantages ideologically moderate candidates because extremists activate the opposing party’s base more than their own.

Right now the Democratic Party is full of rhetoric about needing more progressive candidates to fire up the base, but the evidence remains strong that the winning candidate is usually the one closest to the median ideology of the district. Yes there are lots of people who say they don't vote because the candidates are all the same and nobody is offering them real help, but in real elections nobody can turn out enough of those disaffected voters to make appealing to them a winning strategy.