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Bernie Sanders' supporters have been pushing to nominate former state Sen. Nina Turner of Cleveland for vice president as an alternative to U.S. Sen. Tim Kaine of Virginia.

(Chuck Crow, Plain Dealer Publishing Co.)

PHILADELPHIA--Bernie Sanders supporters have been working to nominate ex-state Sen. Nina Turner for vice president, as an alternative to Hillary Clinton's pick of U.S. Sen. Tim Kaine of Virginia.

But their efforts may have been in vain, as the deadline has expired to officially offer an alternative VP pick to the Democratic National Convention, activists said.

Turner's name was circulated as a potential Clinton running mate by pro-Sanders activists on Facebook. Turner, a Cleveland Democrat who ran unsuccessfully for Ohio secretary of state in 2014, has been among the most vocal supporters of Sanders' presidential campaign.

On Tuesday, officials with the Bernie Delegates Network, a group independent of Sanders' campaign that claims to represent two-thirds of his roughly 1,900 convention delegates, said they recruited an unnamed person as an alternative to Kaine, a moderate Democrat chosen by Clinton to be her running mate.

However, Sanders' backers ran out of time to get the necessary paperwork to enter the alternative candidate's name, said Donna Smith, executive director of Progressive Democrats of America. Smith said Democratic National Committee officials led them on a "wild goose chase" to get the necessary forms.

Nominating a vice president requires signatures from 300 elected delegates, no more of 50 of whom can be from any one delegation, Smith said.

Norman Solomon, a Sanders delegate from California and a national coordinator of the Bernie Delegates Network, refused to say which candidate they planned to enter, as the issue was now "moot." When asked specifically if Turner was the mystery candidate, Solomon declined to comment.

Sanders activists said they never expected their candidate to be nominated over Kaine. Rather, Smith said, they just wanted to offer a symbolic alternative.

"This was to truly reflect the energy that has come from Bernie delegates," she said.

Turner, reached by text message, said she was not involved in the effort and had turned down the offer to become an alternative vice presidential candidate.

Jason Edwards, who heads the Sanders delegation from Ohio, said he heard about plans to run Turner for vice president. But he said Sanders delegates from Ohio weren't behind the effort.

"I don't think that's a fight that we want to take on right now," Edwards said.

Bernie Delegates Network officials also released a non-scientific survey of 318 of Sanders' 1,846 convention delegates, finding that 55 percent are willing to participate in a non-violent protest during Tim Kaine's speech on Wednesday. Twenty-six percent said no, while 18 percent said they weren't sure.

Asked whether they would be willing to non-violently protest Clinton's acceptance speech on Thursday, 58 percent said yes, while 24 percent said no.

This story was updated with a comment from Nina Turner.