At the same rally, one of Mrs. Clinton’s supporters, Mark Cuban, the billionaire owner of the Dallas Mavericks, put it even more bluntly: “There’s going to come a time, if Donald Trump, God help us, was president, where a Putin or an Assad would say to him, ‘Donald, if you do this, I’ll give you $20 billion,’” Mr. Cuban said, referring to President Vladimir V. Putin of Russia and President Bashar al-Assad of Syria.