Robert Satiacum, a delegate from Washington State, said he was unsure whom he would support in November but that he had ruled out Mrs. Clinton, whom he termed a “criminal,” and Mr. Trump, whom he described as a “clown.”

“Never, never,” he said of voting for Mrs. Clinton. “Absolutely not.”

Some were there to press specific issues: Three men and two woman carried a “Natives for Bernie” sign and held forth to a group of reporters about why they viewed Mr. Sanders as the superior candidate for Native Americans.

But the activist holdouts appeared outnumbered by journalists who crowded around with tape recorders and television microphones. A few lines of Pennsylvania state troopers and Philadelphia police officers, clad in blue and gray, watched over the proceedings impassively. Others stood guard outside, blocking the doors to prevent more protesters from joining.