Sanders delegates threaten challenge to Kaine
© Greg Nash

PHILADELPHIA — A group of Democratic delegates backing Sen. Bernie SandersBernard (Bernie) SandersFreedom Caucus chairman: Who was FBI informant reporting to? Trump: ‘Clapper has now admitted there was spying on my campaign’ Overnight Defense: Trump decision on Korea summit coming 'next week' | China disinvited from major naval exercise | Senate sends VA reform bill to Trump MORE (I-Vt.) said Monday that they are actively exploring a challenge to Sen. Tim KaineTimothy (Tim) Michael KaineParent of middle school students amid shooting: ‘This happens in high school, not here’ Kaine demands answers on Pentagon missions in Africa Lawmakers push for House floor debate on war authorization 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 ClintonHillary Diane Rodham ClintonFranklin Graham hosting 10 rallies in California urging Christians to vote Trump: Obama didn’t want to ‘upset the apple cart’ by investigating Russians Rick Santorum: Migrant children the government lost track of aren't lost 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.

Still, Solomon said, "We're not delusional. We understand Tim Kaine is going to be the vice presidential nominee."

The group hinted that there would likely be protests on the convention floor Monday night but didn't go into specifics.

Monday's press conference comes hours ahead of the formal opening of the party's convention in Philadelphia.

Sanders supporters expressed frustration with the national party Monday after WikiLeaks posted emails showing that top DNC officials apparently worked to undermine the Vermont senator's campaign.

Asked about the potential for protests when DNC Chairwoman Debbie Wasserman Schultz us at the podium, Bernal said, “I would be very surprised if you could have a conversation in normal tones.” Wasserman Schultz on Sunday announced that she would resign from her role at the end of the convention.

— This report was updated at 12:35 p.m.