PHILADELPHIA — A coalition of delegates supporting Sen. Bernie Sanders were scrambling for a way to oppose Hillary Clinton's vice presidential running mate on Tuesday night, after Democrats foiled their plan to formally propose an alternative to Sen. Tim Kaine, D-Va.

Sanders backers were plotting from the outset of the convention to oppose Kaine. But the delegates said Democratic National Committee officials stonewalled their efforts to obtain the paperwork needed to submit an alternate candidate.

As a result, some delegates may be planning an effort to voice their displeasure in Kaine's nomination on the floor Tuesday evening, according to Sanders supporters speaking for the Bernie Delegates Network.

That group said it had started an effort to draft a progressive vice presidential alternative by collecting signatures. But because the deadline to oppose Kaine's nomination has passed, a spokeswoman for the network said the delegates were instead planning some other way to show their disapproval, although it's not yet clear what they might try.

The possible effort comes after a straw poll of 318 Sanders delegates conducted by the group on Sunday showed that most wanted to find some way to replace Kaine. Only 19 percent of Sanders delegates said they would not be interested in opposing Kaine's nomination as the Democratic vice presidential candidate.

The rest, 81 percent, said they would be receptive to a progressive alternative on the ticket. The Bernie Delegates Network claims to include 1,250 Sanders delegates present at the convention this week.

While Clinton's team and the DNC were hoping to confine all the Sanders-related anger to Monday, this latest effort makes it clear they aren't going away, and neither are the protests.

Norman Solomon, a Sanders delegate from California, questioned why the media had engaged in an "outcry about rudeness at a convention" after turmoil on the floor Monday evening, instead of focusing on the failures of the country's healthcare system.

"Complacency is inappropriate, that being here for a celebration is totally inappropriate," Solomon said. "We're supposed to unify behind complacency?"

Solomon refused to reveal the name of the vice presidential nominee even under intense questioning from reporters at a briefing Tuesday morning. The briefing grew contentious when Sanders supporters speaking for the network refused to answer basic questions about their failed attempt to collect the 300 signatures and any brewing plans for the roll call vote.

The first day of the Democratic convention was marred by unrest among delegates as they grappled with the news that DNC officials had conspired to stifle Sanders' popularity during the primaries. DNC Chairwoman Debbie Wasserman Schultz was forced to step down from her position amid the backlash over those revelations, which emerged in a massive leak of internal emails published by Wikileaks on Friday.

Michelle Obama, Sen. Elizabeth Warren and Sanders himself each made impassioned arguments in favor of party unity Monday evening as the DNC leak threatened to unravel the sense of solidarity that Clinton's camp had fought so hard to foster.

While delegates welcomed Sanders to the stage with a three-minute standing ovation, they heckled earlier speeches by shouting down each mention of Clinton.

Supporters of the Vermont senator fashioned dozens of makeshift posters by writing his name on Clinton campaign signs that the DNC had distributed. Some of his most ardent fans even wept in the arena as he spoke of the need for Democrats to set aside their differences and vote for Clinton.

But the message from the man behind the movement was clear: Now is the time to fall in line behind Clinton.