Donald Trump, a man who has managed to say horrible things about women, immigrants, black people, Muslims, veterans...well, let's just say nearly everyone, while demonstrating an immense lack of understanding regarding foreign and domestic policy (or even the English language), while also showing he is thin skinned and wildly temperamental, assumes that the only way he can lose an election to be president of the the U.S.A. is because it was stolen from him.
Trump has been laying the groundwork for a "rigged election" for a while. During the third and final presidential debate, Hillary Clinton went through a history of Trump claiming contests were rigged against him, back through the primaries and even into the Emmys. Trump lashed out against the Emmys when his Apprentice reality show did not win an award, as is his style(1).
When the floodgates opened(2) with claims of sexual assaults against Trump, he lashed out against the media(3):
Trump, who spent the week launching tirades at Clinton, journalists and his own party, continued on Sunday to portray himself as the victim. He tweeted: “Polls close, but can you believe I lost large numbers of women voters based on made up events THAT NEVER HAPPENED. Media rigging election!”
In another post, he said: “Election is being rigged by the media, in a coordinated effort with the Clinton campaign, by putting stories that never happened into news!”
Trump's surrogates have gone out and promoted this claim as well, even as Republican officials try to downplay the storm.