Warren vs Biden in the polls: how sudden was the change?

Question

It's been pointed out to me (hat tip to dolphin_of_france) that some recent polling puts Warren ahead of Biden in the Democratic primary voting intention.

How sudden was this change? How much [of this] change happened in the time proximity of the recent Trump-Zelensky phone call disclosures?

Interestingly, as CNN noted about Warren (on Sep 23):

She has also been very outspoken on another issue supported by approximately 70% of Democrats: Impeaching Trump. Warren was the first major presidential candidate to call for impeaching the President back in April, just days after Robert Mueller's report was released. And on Friday after allegations were reported that Trump spoke to the Ukrainian President in July and pressured him to investigate Biden and Biden's son Hunter in an obvious effort to get dirt on Biden, Warren went a step further. She took to Twitter slamming Democrats in Congress for failing to act [...]

CNN then contrasts that with Biden's more cautious approach.

As it's been pointed out to me below, I should also be asking how reliable/relevant these polls are. Apparently they are state-level polls.

Answer 1

The Real Clear Polling Averages are a useful guide here.

The poll in question is a Quinnipiac University poll of 561 registered voters nationwide. (Technically the poll was of 1337 registered voters, but the question on the Democratic Primary was only asked to the 561 respondents who identified as Democrats or Independents who leaned Democratic.)

This poll shows Warren with 27% support compared to Biden's 25% support. Within Quinnipiac's polls, it represents a rapid jump in Warren's support from their last poll at the end of August:

Poll data of "DEMOCRATS/DEMOCRATIC LEANERS" from throughout 2019:

2019:	Sep 25	Aug 28	Aug 06	Jul 29	Jul 02	Jun 11	May 21	Apr 30	Mar 28
Biden	25	32	32	34	22	30	35	38	29
Sanders	16	15	14	11	13	19	16	11	19
O'Rourke	2	1	2	2	1	3	2	5	12
Harris	3	7	7	12	20	7	8	8	8
Warren	27	19	21	15	14	15	13	12	4

However, there is not enough resolution here to determine when the jump occured or to attribute it to any specific effect. Data for this poll was collected from September 19th to 23rd, so it is likely too early to see much effect from the Ukraine scandal, as it was just starting to blow up at the end of the polling period.

Another poll from Economist/YouGov from September 22nd - 24th also shows Warren 1% ahead of Biden. However the Economist/YouGov has long showed weaker Biden leads than other polls, so this doesn't represent a real shift either:

	Biden	Warren
9/22 - 9/24:	26	27
9/14 - 9/17:	26	21
9/8 - 9/10:	26	26
9/1 - 9/3 :	26	22

At the same time, another poll released by Morning Consult with data collected from September 16-22nd shows Biden up by 12%. So, I'd argue that it's still a bit early to say if anything significant has changed in the Biden-Warren race and definitely much too early to say if this scandal will hurt Biden.