Solomon said the party should take note of polling that suggests high numbers of Sanders' supporters are still leery about backing Clinton.
"This fall-off in support is plausibly related to her demonstrable contempt for the progressive wing of this party with the selection of Tim Kaine," he said, citing Kaine's past votes on trade and banking. In recent days, Clinton's vice presidential pick has said he would oppose the Trans-Pacific Partnership (TPP), despite having backed the trade deal in the Senate, but Solomon and many other progressives remain skeptical.
Solomon has helped launch a new organization called the Bernie Delegates Network, which claims to have more than 1,250 delegate members and, despite its name, pledges to be working independently from the campaign.
The group has been conducting straw polls of Sanders' delegates, and plans to survey the bunch again in the next 24 hours about possible protests or even floor action to object to Kaine on the ticket. Solomon suggested actions such as staying seated or turning backs when Kaine takes the stage, but said his team was looking into procedural options to protest the Virginia senator in a more formal way as well.
"The onus for party unity was on Hillary Clinton, and it is a bit much to be told, 'You Bernie delegates better snap to it for party unity,'" Solomon added. He suggested a vote on the TPP on the convention floor as another possible olive branch to progressives.