Al Gore, the former US vice-president, has endorsed Barack Obama as the Democratic party's candidate for president.
Gore, who is to speak at a rally for Obama in the state of Michigan on Monday, said he intends to do "whatever I can" to ensure the Illinois senator is elected president.
"Over the next four years, we are going to face many difficult challenges - including bringing our troops home from Iraq, fixing our economy, and solving the climate crisis," Gore, former vice-president to Bill Clinton, said in a message on his website on Monday.
"Barack Obama is clearly the candidate best able to solve these problems and bring change to America."
Obama in effect clinched the Democratic nomination earlier this month after he captured enough delegates, leading to his rival Hillary Clinton suspending her campaign.
He will face the Republican candidate, John McCain, in November's presidential election.
'Taking action'
Gore, who lost the 2000 presidential election to Republican George Bush, stayed neutral during the often bitter primary race between Obama and Clinton, the wife of his former boss, Bill Clinton.
He is now best known for his work on combating climate change, receiving a Noble Peace Prize for his efforts and producing the Oscar-winning documentary An Inconvenient Truth.
"Over the past 18 months, Barack Obama has united a movement. He knows change does not come from 1600 Pennsylvania Avenue or Capitol Hill, Gore wrote, referring to the White House and congress.
"It begins when people stand up and take action."
Ex-Clinton aide hired
Also on Monday, Obama's campaign said it had hired Hillary Clinton's former campaign manager, Patti Solis Doyle, to serve as chief of staff to the still unnamed vice-presidential candidate.
Doyle, who had worked for Clinton since her time as first lady in the White House in the 1990s, was fired after the New York senator's disappointing performance in the so-called "Super Tuesday" primaries on February 5.