These charts reflect the results of the new CNN / SSRS Poll - done between August 15th and 18th of a national sample of 402 Democrats and Democratic-leaning Independents who are registered to vote. The margin of error for the survey is 6.1 points.
The two most important changes in this poll recently regard the candidacies of Joe Biden and Kamala Harris. Biden only led by 5 points in June, but has increased his lead to 14 points in August. Harris was at 17% in June (in second place only 5 points behind Biden). She has dropped 12 points, and now sits in a tie for fourth place with Buttigieg at 5%.
The second chart shows what those voters consider most important. While 54% say the ability to beat Trump is most important, only 39% say it's important for the candidate to agree with them on issues. This benefits Biden, since he's generally regarded as the candidate who could most easily beat Trump.
NOTE -- I only included the candidates in this chart who got at least 2% in the poll.