US Senator Bernie Sanders has cleared the way for former vice president Joe Biden to go head-to-head with Donald Trump in November’s critical general election.

Mr Sanders announced his decision to exit the 2020 presidential race on Thursday morning (Australian time), conceding “the path toward victory is virtually impossible”.

The 78-year-old Vermont senator held an all-staff conference call just hours prior to formally end his five-year-long quest for the White House. 

It leaves Mr Biden the presumptive Democrat nominee to take on the president. 

Mr Biden credited Mr Sanders for creating “a movement” based on ideas and drawing attention to injustices and inequalities faced by his many progressive supporters. 

Senator Sanders and his supporters have changed the dialogue in America.’’
-Joe Biden after Bernie Sanders’ announcement

He called on progressives to “join us”, adding “you’re needed”.

“I see you, I hear you, and I understand the urgency of what it is we have to get done in this country,” Mr Biden said.

In a video message, Mr Sanders congratulated Mr Biden before thanking an “unprecedented grassroots political campaign” that relied entirely on small donations from millions of his supporters.

Today I am suspending my campaign. But while the campaign ends, the struggle for justice continues on. https://t.co/MYc7kt2b16

— Bernie Sanders (@BernieSanders) April 8, 2020

He spoke of a “difficult and painful” decision, saying Mr Biden is “a very decent man” who he will continue to work with “to move our progressive ideas forward”.

Let us go forward together. The struggle continues.

— Bernie Sanders (@BernieSanders) April 8, 2020

Mr Sanders, a democratic socialist whose progressive agenda pulled the party sharply to the left, also mounted an unexpectedly strong challenge in 2016 to eventual Democratic nominee Hillary Clinton.

After shooting to an early lead in the race, he faded quickly after losing South Carolina in late February and has been under pressure to halt his campaign after Biden won resounding victories in primary contests on March 17 in Florida, Arizona and Illinois.

The departure of Mr Sanders, the last remaining rival to Mr Biden, sets up a battle between the 77-year-old former vice president and Republican President Trump, 73, who is seeking a second four-year term in office.

-with AAP