A new national Quinnipiac poll shows Bernie Sanders jumping out to his biggest lead in any national poll of the Democratic primary:

Sanders 25

Biden 17

Bloomberg 15

Warren 15

Buttigieg 10

Klobuchar 4

Just before the Iowa caucuses, the topline numbers in Quinnipiac’s late-January national poll were:

Biden 26

Sanders 21

Warren 15

Bloomberg 8

Klobuchar 7

Buttigieg 6

Biden has been counting on South Carolina as his firewall (where two-thirds of Democratic primary voters are black), but the new national Quinnipiac poll shows support for Biden among African Americans dropping from 52 percent to 27 percent since the Iowa caucuses.

Quinnipiac’s results among African-American Democratic primary voters in late January:

Biden 52

Sanders 15

Bloomberg 8

Warren 7

Buttigieg 0

And now:

Biden 27

Bloomberg 22

Sanders 19

Warren 8

Buttigieg 4

Bloomberg, who gained the most among black voters, isn’t competing in the South Carolina primary — so Biden still might have a shot there to turn things around — but poor showings in New Hampshire and Nevada could nevertheless drag down the former vice president ahead of the first-in-the-South primary.