This chart shows the results of the four latest national Democratic presidential polls (from RealClearPolitics). All four of them show Clinton still maintains a lead nationally -- with margins from 5 to 18 points. They are:
Quinnipiac University Poll (March 16-21) 635 registered voters (3.9 point moe)
Monmouth University Poll (March 17-20) 391 registered voters (5.0 point moe)
CBS News / New York Times Poll (March 17-20) 388 likely voters (6.0 point moe)
CNN / ORC Poll (March 17-20) 397 registered voters (5.0 point moe)