Bernie Sanders urged reluctant supporters today to vote for his Democratic rival Hillary Clinton in order to bar Donald Trump's path to the White House, slamming the Republican as a "demagogue."

"We have got to defeat Donald Trump. We have got to elect Hillary Clinton and Tim Kaine," the Vermont senator told a rally held hours before the opening of the Democratic National Convention.

"Trump is a bully and a demagogue," said Sanders whose call to support party nominee Clinton in November was met with loud jeers and cries of "We want Bernie!" from the fired-up crowd.

The four-day confab in Philadelphia got off to a rocky start following a major internal row over leaked emails that showed party leaders sought to undermine Sanders in the party primaries. 

Many of the die-hard Sanders supporters who descended on Philadelphia loathe the party flag bearer Clinton so much they are unwilling to back her -- even if the cost is a Trump presidency .

Sanders has officially endorsed his rival and has been offered a prime speaking slot later today at the convention, during which he will stress that the party mission is to prevent Trump from becoming the 45th president of the United States.