PHILADELPHIA (AP) - A California delegate says there are serious discussions underway to challenge Hillary Clinton's pick of Tim Kaine as her running mate.

Norman Solomon, a delegate who supports Bernie Sanders, says there is talk among Sanders' delegates of walking out during Kaine's acceptance speech or turning their backs as a show of protest.

Solomon said he believes a "vast majority" of Sanders delegates support these kinds of protests to express their dismay. Sanders' supporters say they are concerned that Kaine is not progressive enough.