… Early national polls conducted in the year before the first nominating contests do have predictive value, but remember there isn’t one national primary. Instead, parties choose presidential nominees via state-by-state elections over the course of a few months. And that sequential nature of the primary makes it hard for national polls to capture all the dynamics of the race — a candidate falling short of expectations in Iowa or unexpectedly winning both Iowa and New Hampshire can swiftly alter the playing field for the remaining primaries.
But having examined all the national polls from the last six months of 2019, the bottom line is that, at this point, Biden remains the favorite to win the Democratic nomination. CONT.
Geoffrey Skelley, FiveThirtyEight