At the time, it was a bombshell that neither Obama nor McCain -- a fellow Republican -- was expecting.
Powell barely knew Obama, the junior senator from Illinois, but he had been watching his presidential campaign carefully and was impressed by what he called Obama's "intellectual vigor." His assessment of the McCain campaign was far less charitable, from selecting Sarah Palin to serve as his running mate to what he viewed as a scattershot approach to the economic crisis.
"It isn't easy for me to disappoint Sen. McCain in the way that I have this morning," Powell said in an interview on NBC's "Meet the Press," where he delivered the news that helped shape the final days of the campaign.
In the long public life of Colin Luther Powell, who died Monday at the age of 84, that decision offers a brief, yet important window into his view of the American presidency. He saw the office from the highest corridors of power, serving in three Republican administrations, but he believed Obama represented the future.
"He is a new generation coming into the world, onto the world stage," said Powell.
In a statement on Monday, Obama remembered Powell as "an exemplary soldier and an exemplary patriot."
"Everyone who worked with General Powell appreciated his clarity of thought, insistence on seeing all sides, and ability to execute," Obama said. "And although he'd be the first to acknowledge that he didn't get every call right, his actions reflected what he believed was best for America and the people he served."
He recalled Powell's 2008 endorsement and said that he was not only impressed by Powell's willingness to cross the political divide, but grateful for calling out those who had questioned his roots.
"At a time when conspiracy theories were swirling, with some questioning my faith, General Powell took the opportunity to get to the heart of the matter in a way only he could," Obama recalled. "'The correct answer is, he is not a Muslim; he's a Christian,' General Powell said. 'But the really right answer is, 'What if he is?' Is there something wrong with being a Muslim in this country? The answer's no, that's not America. Is there something wrong with some seven-year-old Muslim-American kid believing that he or she could be president?'"
Obama added, "That's who Colin Powell was. He understood what was best in this country, and tried to bring his own life, career, and public statements in line with that ideal."
Powell, who served as the nation's first Black secretary of state and the first and only Black chairman of the Joint Chiefs of Staff, bristled at the suggestion that he endorsed Obama because he would be the first Black president. Instead, he explained at the time and over the last decade, he supported Obama because he didn't like the darkening political discourse in America.
"We have come to live in a society based on insults, on lies and on things that just aren't true," Powell said in 2018. "It creates an environment where deranged people feel empowered."
Powell would go on to endorse Hillary Clinton in 2016 and Joe Biden in 2020 -- both longtime acquaintances and friends in Washington -- as he continued to speak out against the changing face of the Republican Party.
Today, his voice was hardly alone among party elders amid the rise of Donald Trump.
But 13 years ago this week, when Powell unveiled his support for Obama, the weight of his words shook the presidential campaign in a way that few other endorsements did. He did not hit the campaign trail for Obama in the final days of the race, but simply made his views clear.
"A great soldier, a great statesman and a great American has endorsed our campaign to change America," Obama said on that day, after learning the news.
Looking back, even McCain's reaction is astonishing in today's vitriolic politics.
He told advisers and friends at the time that he was deeply disappointed, but said only these words publicly: "I respect and continue to respect and admire Secretary Powell."