Washington (CNN) For months, national polls of the Democratic presidential primary showed former Vice President Joe Biden as the most popular candidate. Then he took a self-described "gut punch" in Iowa and left New Hampshire before the primary results were even delivered.

Now, after finishing fourth and fifth in the first two contests, Biden's campaign is seeking a rebound in the next two contests in South Carolina and Nevada. But his supporters are shaken and other candidates, including former New York City Mayor Mike Bloomberg, are seeking to chip away at his strength -- African American voters.

Nowhere is Biden's sudden struggle more clear than in Congress, where the former vice president has earned far more endorsements among Democratic candidates than any other but has not received any since Iowa — and seen potential high-profile supporters consider others.

"Obviously fourth and fifth place isn't where you want to be," said Virginia Rep. Bobby Scott, who has not endorsed in the primary. "His campaign is based on electability. It's kind of hard to make that case when you're coming in fourth and fifth place."

It's clear that the first two contests have helped Bloomberg, who chose not to campaign in the first four races and to instead focus on March 3 -- Super Tuesday -- when more than a third of the delegates are awarded. Bloomberg has appeared to show some appeal to black voters. A Quinnipiac poll released on Monday showed that Biden's support among black voters had fallen since late January, from 48% to 27%, while Bloomberg's support has risen from 7% to 22%. Since Iowa, seven House Democrats have announced they're supporting Bloomberg, including members from swing districts and the Congressional Hispanic and Black caucuses.

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