… In soccer, the most obvious advantage of having a man advantage after an opposing player has been sent off on a red card is that it’s much easier to score goals. But what’s nearly as important is that it becomes very difficult for the other team to score a goal; defense becomes a cinch.
So it also goes in congressional races when your party is having a wave election: You’ll win plenty of your opponent’s seats, but — not to be overlooked — you’ll usually also lose almost none of your own seats. The GOP didn’t lose any of its own Senate seats in 1994, 2010 or 2014, for example. Democrats didn’t lose any of their own seats in 2006 or 2008. Between these five election cycles, the “waving” party went undefeated in defending its Senate seats.
This year, however, we have a seeming contradiction: The polls are pointing toward a wave in the House, with an average projected gain of 35 to 40 seats for Democrats and a popular vote win of 8 to 10 points. And yet, Democrats are at risk of losing several of their own Senate seats, which could offset any gains they may make among GOP-held seats and make it much harder for Democrats to take control of the Senate. CONT.
Nate Silver, FiveThirtyEight