US President Barack Obama waves to guests as he walks to board Marine One at the White House in Washington, DC, on August 31, 2016 as he departs for an environmental summit in Lake Tahoe, Nevada. / AFP PHOTO / NICHOLAS KAMMNICHOLAS KAMM/AFP/Getty Images

US President Barack Obama waves to guests as he boards Marine One at the White House in Washington, DC, on August 31, 2016 as he departs for an environmental summit in Lake Tahoe, Nevada. / AFP PHOTO / NICHOLAS KAMMNICHOLAS KAMM/AFP/Getty Images

US President Barack Obama walks to board Marine One as he departs the White House in Washington, DC, on August 31, 2016 for an environmental summit in Lake Tahoe, Nevada. / AFP PHOTO / NICHOLAS KAMMNICHOLAS KAMM/AFP/Getty Images

A supporter takes a photo of Republican presidential nominee Donald Trump during a campaign rally in Everett, Washington, U.S., August 30, 2016. REUTERS/Carlo Allegri

Supporters of Republican presidential nominee Donald Trump cheer during a campaign rally in Everett, Washington, U.S., August 30, 2016. REUTERS/Carlo Allegri

Supporters of Republican presidential nominee Donald Trump cheer during a campaign rally in Everett, Washington, U.S., August 30, 2016. Picture taken August 30, 2016. REUTERS/Carlo Allegri

Republican presidential nominee Donald Trump speaks during a campaign rally in Everett, Washington, U.S., August 30, 2016. Picture taken August 30, 2016. REUTERS/Carlo Allegri

Republican presidential nominee Donald Trump speaks during a campaign rally in Everett, Washington, U.S., August 30, 2016. Picture taken August 30, 2016. REUTERS/Carlo Allegri