DENVER—Bill Clinton walked onto the stage of the Pepsi Center on Wednesday night to sustained, roaring and amorous applause. Instead of signs with the names of Obama or Hillary, the hall waved American flags. Every time the ovations seemed to subside, they roared back up again. The time clock above the teleprompter froze at 10 minutes. With the tribute not eating into his speech, Clinton let them applaud. Not that he could have stopped them if he wanted.