By Kevin Connolly
BBC News, Washington
It is getting harder and harder to remember the last time anything happened in Campaign 2008 that John McCain's camp would regard as clear, unambiguous good news.
Colin Powell was himself once tipped as a presidential candidate
Perhaps it was the heady moment just after he made Governor Sarah Palin his running mate and just before her "gosh-darn" stump speech began to grate on the American ear.
Since then Mr McCain has struggled to find the right tone and the right message amid a sinking economy, and Mrs Palin has come dangerously close to running aground in a series of excruciating interviews on network television.
Opinion polls have also provided depressing evidence that Barack Obama is winning the arguments and - for the moment at least - winning the election.
Now Mr McCain is losing the battle for high profile endorsements too.
Colin Powell, the latest big figure to back Mr Obama, might have had a few kind words for his old friend and party colleague - but he's still voting for the Democrat.
It is a decision which will come as a particular blow to John McCain. Mr Powell is, after all, a Republican, a lion of the party and a retired general with impeccable national security credentials.
He served as chairman of the Joint Chiefs of Staff during the first Gulf War in 1991 and was then Secretary of State during George W Bush's first term in office.
He is a big player in Washington and a contemporary of Mr McCain.
His view that America needs the generational change which Mr Obama is in a position to offer will undermine his old friend's credibility.
Given that the US is due to vote on 4 November and that we have already reached the point in the year when stores are beginning to erect their Christmas displays, Mr Powell can hardly be accused of rushing his decision.
Even the way the endorsement came suggests that the general was rather torn.
The man who has served as America's most senior soldier and as its most senior diplomat thinks Obama is ready to lead the nation
It was offered during a thoughtful TV interview rather than on stage at an Obama rally. It is particularly damaging for the Republicans that Mr Powell suggested Sarah Palin's inclusion on the ticket and the negative tone of the campaign were major factors in his decision.
The implication is clear - that his own party could have won his support, but failed to do so.
You might argue in fact that in making up his mind so late in the campaign Mr Powell is demonstrating a political version of the so-called "Powell doctrine" he practiced as a military man.
That essentially involves only making your move when you are certain that overwhelming force is on your side. Or, in this case, that you are on the side of overwhelming force.
So, given that this is not an act of reckless political courage, does a Powell endorsement have real value at this stage of the race?
I think it does, although it certainly won't be decisive.
The first way in which it matters is simply that it means another day of positive headlines for the Obama campaign, and by extension another day in which the news cycle spins out of the control of the McCain camp.
There will be big headlines too, in the American media.
Colin Powell has been a high profile Republican since leaving the army
Mr Powell is a hugely respected and high-profile figure - if he says he is disappointed with the tone of the Republican campaign or that Sarah Palin simply isn't ready to be president, then that will be widely reported.
It is also true that Mr Powell's views probably carry quite a bit of weight with the type of military veterans who are natural McCain voters and who distrust Obama - in part at least - because of his lack of international experience.
If the man who has served as America's most senior soldier and as its most senior diplomat thinks Obama is ready to lead the nation, then that might be enough to convince a few waverers.
There is a rather touching generational resonance in all this too - it once seemed possible that Colin Powell himself would become the first African-American President of the United States.
He was born in an America where it wouldn't have been possible for a black man - however brilliant and inspiring - to reach the position in which Barack Obama now finds himself.
Now he is in a position to lend his authority to an African-American politician who may be about to complete an astonishing journey on which he himself took some of the first steps.
Mr Obama is, in Colin Powell's words, a transformational figure.
It is worth noting though that the Obama camp will want to be sure that no-one sees Mr Powell's decision as some kind of act of racial solidarity.
Powell criticised the choice of Sarah Palin as McCain's running mate
They want everyone to be clear that this is all about a senior figure in American politics endorsing the judgment and readiness to lead of a man who he has met and engaged in serious discussion.
Colin Powell of course, remains a Republican - and he won't be campaigning for Obama.
But it is hard not to wonder about his decision to go against the interests of his own party in this very public way.
Could it be rooted in the bitter experience he suffered as secretary of state when he became the front man for the Bush administration, charged with selling the dossier on Iraq's weapons of mass destruction to a dubious world?
Still, the endorsements of the big players in American politics do matter, and Mr Obama has simply been getting more of them - not just from major political figures but from big newspapers too.
It's no great surprise that the Washington Post threw its weight behind Barack Obama. But the Los Angeles Times is also endorsing the Democrat, the first time the west coast newspaper has backed any candidate since it went for Richard Nixon back in 1972.
Given how that particular recommendation turned out you can see why the Times stayed out of the endorsement business for 36 years, although it is important to bear in mind that in 1972 the Vietnam war was still raging, that no-one had yet heard of Watergate and that Mr Nixon was from California.
You can bet the LA Times came to regret that endorsement of Nixon.
And you might remember that Colin Powell himself once backed Dick Cheney's run for high office with the view that he would make a "superb" vice-president.
I wonder if that's the word he would now use to describe Mr Cheney's time in office.