Former US President Barack Obama has endorsed his vice president Joe Biden's presidential campaign in an effort to bring the Democratic Party together ahead of the election later this year.

After staying on the sidelines as a record high number of Democratic candidates fought for the right to take on President Donald Trump in the 3 November election, Mr Obama gave his support to Mr Biden in a video posted online.

"Choosing Joe to be my vice president was one of the best decisions I ever made and he became a close friend. And I believe Joe has all the qualities we need in a president right now," Mr Obama said in the video.

The endorsement by the first black US president came after Mr Biden's last remaining rival in the Democratic nomination race, Senator Bernie Sanders, backed Mr Biden yesterday.

Mr Obama had publicly remained neutral as Mr Biden, who served him for eight years in the White House, at first struggled to impose himself on the 2020 Democratic race then swept to victory in a series of primaries last month.

Mr Obama "is a uniting partner in helping bridge this party after an intense primary and paving the road ahead," said Democratic strategist Antjuan Seawright.

Three years after they departed the White House, Barack Obama and his wife Michelle remain two of the most beloved figures in the Democratic Party.

President Trump won in 2016 vowing to reverse many of the two-term president's policies.

Mr Obama's popularity within the Democratic Party could bolster Mr Biden, particularly among demographic groups with which he has struggled.

A CNN poll released when Mr Obama left office showed more than 70% of voters under the age of 34 viewed him favourably as did 90% of self-described liberals.

"Because he is so popular and the comparison between President Trump and Barack Obama is so stark, it will be such a unifying, motivating factor," said Terry McAuliffe, a former Virginia governor and longtime party official.

The former president's support also gives Mr Biden access to a network of donors and other allies, some of whom were initiallycool to the former vice president.

"If in fact I win, get ready to suit up," Mr Biden told a group of former Obama officials at a fundraiser in Washington in November.