PHILADELPHIA — A group of Democratic delegates backing Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) said Monday that they are actively exploring a challenge to Sen. Tim Kaine Timothy (Tim) Michael KaineNames to watch as Trump picks Ginsburg replacement on Supreme Court Barrett seen as a front-runner for Trump Supreme Court pick Biden promises Democratic senators help in battleground states MORE’s (Va.) vice presidential nomination.

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"There's serious interest right now and exploration as we speak of a formal challenge with an alternative candidate," said Norman Solomon, a Sanders delegate and national coordinator of the Bernie Delegates Network.

The group, which is independent of the Sanders campaign, said it was actively polling its members, which include 1,250 of Sanders’s 1,894 delegates.

The group released a poll earlier this month that found that less than 3 percent of its supporters found Kaine acceptable.

Solomon said he hoped the Democratic National Committee (DNC) releases information within hours on how to submit a challenger to Kaine, which he said would require the signatures of 300 delegates.

He said they have until Wednesday morning to file a challenge to Kaine and stressed that while his group would take any requests from the Sanders campaign under consideration, the delegate group is an independent organization.

Karen Bernal, a Sanders delegate from California, said presumptive Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE needs to "walk the walk" with the progressive wing of the party.

"Her choice for Tim Kaine speaks so much louder than anything she's said on the campaign trail," she said.