Actually he has. I generally don’t try to self-promote in other people’s comment sections, but I wrote about this exact subject earlier this week.
The link you provide cites the Quinnipiac poll, which shows Warren at 27 percent. Polls where Warren has such high numbers are outliers and they tend to have extremely low sample sizes (n=<700). If you look only at polls in the last month with sample sizes above 1000 (1200 is standard), then Sanders and Warren are tied at 16 points.
Morning Consult, which has the highest sample sizes in the industry (7,000–16,000) has had Sanders holding steady between 19 and 23 since January. Warren surpassed him in the last one (21–19), but the margin of error is (+/-1), so when comparing any two candidates, you have to double it.
In other words, technically by any measure and looking at any poll, Sanders and Warren are in a statistical tie.
Also, Warren’s base is whiter, older and more educated the Bernie’s, so polls of “likely voters” tend to overcount her support.
