Bernie Sanders hasn't picked up the voters who are deserting Joe Biden, but he's the clear beneficiary of the former vice president's rapid collapse.

The big picture: Of the top six candidates in the race, Sanders' polling numbers have changed the least over the last few weeks — but Biden's fall has made Sanders the biggest winner, since the moderate vote is now splintered four ways.

By the numbers: Biden's national polling cratered, from 28.7% on Jan. 27 to 19.2% as of Feb. 12, with most of the damage done after his fourth-place finish in the Feb. 3 Iowa caucus, according to the RealClearPolitics average.

Mike Bloomberg and Pete Buttigieg climbed 6.5 and 3.5 points, respectively, in that time. (Elizabeth Warren dipped 2.6 points and Amy Klobuchar edged up less than a point.)

But the large, moderate voting bloc that propelled Biden is now splitting between multiple candidates.

Between the lines: Biden has lost his electability aura on top of his standing in the polls. He dropped from 29% to 17% in a Morning Consult poll this week that asked Democratic voters who they considered as having the best chance of beating Trump.

That shift put him in third place behind Sanders (29%) and Bloomberg (25%).

One key stat: Sanders has lower polling numbers than any of at least the past five primary front-runners — Democrat or Republican — at this point in the cycle, according to RCP data.

What to watch: Buttigieg is riding the momentum of winning Iowa and finishing a close second to Sanders in New Hampshire. But he needs money to keep going. He is not expected to perform as well with South Carolina's black voters, either.