"The erosion comes mostly among Democrats, a record 20 percent of whom disapprove."

[ProSense]

18. Does this mean

"The erosion comes mostly among Democrats, a record 20 percent of whom disapprove."

...80 percent approve?

The NBC/WSJ has always showed low approval of the President, which is why the intro to the OP snip is phrased as a question. Here is the next paragraph:

On average, little change in either direction Bloomberg's polling in the past two years has generally produced results slightly more positive toward Obama than the average of all public polls, including NBC/WSJ. Other recent surveys of the president's approval rating put him somewhere in the middle -- the latest Rasmussen and Gallup trackers give Obama a 45 percent approval rating, while surveys released this week by CNN and the Democratic firm PPP have him at 43 percent. HuffPost Pollster's average currently puts Obama's approval rating between 43 and 44 percent, almost unchanged since the beginning of 2014.

Here are the NBC/WSJ approval numbers since September 2013:

Sept: 45
Early Oct: 47
Late Oct: 41
Dec: 42
Jan: 42


In fact, if you compare the month before the 2012 election to the current, here is how it stacks up:

Oct. 2012

Very/Somewhat Positive 49 percent
Very/Somewhat Negative 43 percent
Neutral 8 percent

Mar. 2014

Very/Somewhat Positive 41 percent
Very/Somewhat Negative 44 percent
Neutral 15

The biggest jump is in the "neutral" category. "Negative" has been consistent.

http://msnbcmedia.msn.com/i/MSNBC/Sections/NEWS/A_U.S.%20news/US-news-PDFs/14133%20MARCH%20NBC-WSJ%20Poll%20(3-12%20Release).pdf


Edit: still working on that coffee.