A top adviser to Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.) said Thursday that the Democratic presidential primary electorate is still fluid and that the outlook could shift back in Sanders's favor after a swing of damaging losses to former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE.

In an interview on Hill.TV's "Rising," Chuck Rocha said that Biden's sudden surge following an impressive showing in South Carolina's primary could be duplicated by Sanders, who trails the front-runner by fewer than 200 pledged delegates.

"Things can happen quickly...we were all wrought back on our heels because I had never seen that in my 31 years of doing this work. But that's exactly why he wants to debate this weekend," Rocha said. "It's to say if things can go that quickly in that direction, they can happen as swiftly coming back in the other direction."

The Vermont senator and Biden are set Sunday for their first one-on-one debate, which will be held in Washington, D.C., without a live audience or members of the media in the spin room afterwards as precautions meant to prevent the spread of coronavirus, which was declared a pandemic by the World Health Organization this week.

Allies of Sanders are hoping that an impressive performance against Biden in Sunday's debate could swing momentum back in the senator's favor as the primary season moves back into territory seen as favorable for his campaign.

Biden took 10 of 14 states on Super Tuesday and added to his delegate lead this week with wins in Michigan, Mississippi and elsewhere.