Something Funny Happened In Iowa, And It May Hurt Democrats In 2016

538 reports:

Republican Sen.-elect Joni Ernst easily won her race in Iowa last Tuesday, beating Democrat Bruce Braley by 8.5 percentage points. Her victory wasn’t shocking, but its size was (to everyone except pollster Ann Selzer, that is). The final FiveThirtyEight projection had Ernst winning by just 1.5 percentage points.

What the heck happened?

Nate Silver's operation looks at the situation.