The candidate disappointed with US Democratic primary
Mr Sanders, who ran against Hillary Clinton for the Democratic nomination, said in a statement that Mr Trump had tapped into the anger of a declining middle class that was sick and exhausted of establishment economics, establishment politics and establishment media.
"People are exhausted of working more to earn less, to see honorable jobs from China and other low wage countries, that billionaires do not pay their federal taxes and cannot pay the higher education their children, only to the very rich get richer, "he wrote".
Mr Sanders said he was prepared to work Mr Trump on some issues.
"To the degree that he pursues racist, sexist, xenophobic and anti-environment policies, we will vigourously oppose him".
In the Politicus Presidential Projection Maps, I noted that the only way for Donald Trump to win the White House was if Democrats didn't vote, and that is exactly what happened.
Recognizing defeat after this tough battle, Bernie Sanders had called in July on his supporters to rally behind Hillary Clinton against Donald Trump, the "demagogue".
Bernie Sanders set aside his grievances with Hillary Clinton during the general election, crisscrossing the nation on behalf of his ex-primary rival.
"The candidate who ran against special interests is endorsing the candidate who embodies special interests", his campaign said in a release.