US presumptive presidential candidate Barack Obama has picked up the endorsement of former Vice President Al Gore, getting a boost from one of the Democratic Party's most-respected figures as he works to unify party ranks after a divisive primary campaign.

"For America to lead the world through the dangers we're facing, to seize the opportunities before us, we've got to have new leadership. Not only a new President, but new policies. Not only a new head of state but a new vision for America's future."

Gore made the remarks before 20,000 people at a raucous rally at Detroit's Joe Lewis Arena.

Obama acknowledged the gesture, expressing his appreciation for the whole-hearted support from the former vice president.

"I want to thank Al Gore for everything that he is doing to ensure we leave a planet that is better for our children."

In 2000, Gore won the popular vote but lost the disputed election to George W. Bush.

Since then, he has made combating global warming his signature issue, and has been recognized worldwide for his effort - from an Academy Award to the Nobel Prize.