PHILADELPHIA — Supporters of Sen. Bernie Sanders were hoping to send a message of disapproval at the Democratic National Convention Tuesday evening by putting forward a "genuine progressive" candidate for vice president. But the groups behind the attempt say they are giving up, thwarted in their efforts by red tape from the Democratic National Committee.

Many progressives have been dismayed by presumptive Democratic nominee Hillary Clinton's choice of Sen. Tim Kaine, D-Va., as her running mate, saying he is too cozy with Wall Street and on the wrong side of the debate over free trade, having seemed to support at least portions of the Trans-Pacific Partnership agreement before recently saying he'd oppose it.

Donna Smith, executive director of Progressive Democrats of America – which with the independent Bernie Delegates Network spearheaded the effort to put forward an alternative VP candidate – described her frustration to reporters Tuesday morning as "heartbreaking," especially in the face of supposed overtures from the Clinton campaign and the DNC to bring Sanders' supporters into the fold.

"We were sent on a wild goose chase to go after the forms," Smith said, explaining that after a visit to the DNC's headquarters near the convention center in downtown Philadelphia, she was given a "general information email address" and was otherwise ignored.

"If we would have been able to get the forms by this morning, we would have started to circulate them for signatures," Smith said. "We ran out of time."

To enter a name for nomination, convention rules require 300 signatures from delegates, no more than 50 of whom can be from the same state.

Norman Solomon, a Sanders delegate from California and coordinator of the Bernie Delegates Network – which claims to represent around 1,250 of Sanders' 1,900 delegates to the convention – said some 80 percent of the 350 delegates who responded to a questionnaire said they would support putting forward a progressive alternative to Kaine on the floor.

He stipulated, however, that most of the poll's responses came Sunday night and Monday, before Sanders asked his supporters to avoid overt protests on the arena floor and gave a speech proclaiming Clinton the obvious choice of "any objective observer" in November's election against Donald Trump.

Solomon said progressives had identified a candidate willing to stand for the nomination – which he admitted would be a "symbolic protest" rather than a meaningful challenge to Kaine – but "the DNC shut us down."

He described the candidate as someone who was not an elected official, but refused to reveal the person's name and drew derisive comparisons from reporters to conservatives' failed mission to recruit a last-ditch challenge to Trump.