Washington             -               Joe Biden, whose life has been marked by triumph and tragedy, is on the brink of winning a prize that has eluded his grasp on two previous occasions -- the Democratic presidential nomination. The 77-year-old moderate Democrat moved a step closer to heading the party ticket against Donald Trump in November with a series of primary wins on Tuesday over the leftist Bernie Sanders.    One of the youngest senators in US history, vice president for eight years to Barack Obama, America’s first black president, Biden has had a storied political career. His two previous White House runs -- in 1988 and 2008 -- ended in humiliation, however, as the then senator from Delaware failed to win a single state primary or caucus. The second time around, Obama became the Democratic nominee, and selected Biden to be his running mate. Despite being the favorite of the Democratic establishment, the latest White House bid by Biden also looked like it was headed for disaster after a series of disappointing losses to the fiery Sanders in Iowa, New Hampshire and Nevada. But Biden came roaring back in South Carolina’s primary on the strength of overwhelming backing from African-American voters, a crucial base of Democratic support. He followed that up with wins in 10 of the 14 states which voted on March 3 and added to his tally with big victories over Sanders on Tuesday in Mississippi, Missouri and Michigan.