… We know that the first Democratic debate changed the trajectory of the presidential primary. What we’re still trying to figure out is how much it changed the race, and why. For that, The Trailer worked with YouGov Blue and Data for Progress, a liberal advocacy group that has not endorsed any candidate for president. The Post’s polling team vetted their research so we could use it here.
The pollsters went into the field before the debate, finding mostly what other pollsters have found, though their sample had an unusually strong level of support for Sen. Elizabeth Warren (D-Mass.). After 1,522 completed voter interviews, focused on registered Democrats who were likely to vote in the primary, pollsters found Joe Biden in the lead, but narrowly. …
Then came debate night. Over the next few days, opinion sifted, leaving the same candidates in the front but showing Harris surging and Biden falling into a statistical tie with Warren. CONT.
David Weigel, Washington Post