"After eight years of incompetence, neglect and failure, we need change," Gore said. "After eight years when our Constitution has been dishonored and disrespected, we need changes."
In 2000, Gore won the popular vote but lost the disputed election to George W. Bush, who captured Florida and its electoral votes after a divided Supreme Court ended the re-count of ballots.
Since then, Gore has made combatting global warming his signature issue, and has been recognized worldwide for his effort - from an Academy Award to the Nobel Prize.
Obama stoked lasting Democratic anger over the 2000 outcome when he recognized Gore as "the winner of the popular vote for president."
"You remember that," Obama said as the crowd of 20,000 erupted in raucous applause.
Gore is one of the most popular figures in the Democratic Party, but he stayed out of the primary campaign.
It's the second time Obama has rolled out a major endorsement in Michigan, where he did not campaign during the primary because its election violated the party rules. Obama is counting on a win in Michigan in November, but brought Gore and 2004 vice presidential nominee John Edwards to help validate him among Democrats in the state after skipping their primary.
"I'm grateful Al Gore came to Detroit tonight," Obama said.
"But I'm ever more grateful for everything he's done in the last 40 years for this country."
Gore announced his decision in a fundraising e-mail earlier Monday. "From now through Election Day, I intend to do whatever I can to make sure he is elected president of the United States," Gore wrote.
Gore told the rally that the Democratic primary was a contest between a strong and inspiring field of candidates, but "now we've made our choice."
He said Republican candidate John McCain deserves respect for all he has done for this country and for his willingness to debate his party on climate change and other critical issues. But he said McCain's "age and experience" aren't the same as Obama's judgment, noting the Democrat's early opposition to the Iraq war.
He said Republicans criticized President Kennedy for being too young and inexperienced to be president as well, but Kennedy noted that Thomas Jefferson, George Washington and Christopher Columbus all accomplished great things before they reached their mid-40s.
"I feel your determination after two terms of the Bush-Cheney administration to change the direction of our country," he said.
He accused Bush of myriad missteps, including a botched response to Hurricane Katrina, economic problems, foreign policy mistakes and allowing lead-tainted toys and poisoned pet food in from China.
"Even our dogs and cats have learned that elections matter," he said. "This election matters more than ever because America needs change more than ever."
The Republican Party pointed out that Gore's 2000 running mate, Sen. Joe Lieberman, has since left the Democratic Party and become an independent and is backing McCain.
In response to Gore's harsh critique of Bush, GOP spokesman Alex Conant said, "This election isn't about changing the past, it's about changing the future. It's telling that half of the 2000 Democratic ticket endorsed John McCain early in the campaign, while the other half waited until Barack Obama had been the presumptive nominee for weeks." Obama clinched the nomination on June 3.
