US election: what impact do celebrity endorsements really have?

Celebrities are always part of the show in the US presidential election. This is by no means a new trend. Historians have traced the role of celebrities in politics back to the 1920 election, when Warren Harding was endorsed by film stars including Lillian Russell. In 1960, John F. Kennedy was endorsed by Rat Pack members Sammy Davis junior and Dean Martin. More recently, Oprah Winfrey, George Clooney, will.i.am, Brad Pitt and Samuel L. Jackson supported Barack Obama. Actor Clint Eastwood, however, endorsed Republicans John McCain in 2008 and Donald Trump this time around. The 2016 election is no different. So how much of a difference, if any, do high-profile endorsements make? And to which demographics?