Former secretary of state Colin Powell is expected to use a high-profile television appearance tomorrow to end months of speculation by throwing his support behind Barack Obama.

His decision to go on television just two weeks before the election suggests he has finally made up his mind.

Powell, a four-star general who worked closely with three Republican presidents, said he would decide between Obama and John McCain, a long-time friend, after watching the presidential debates, the last of which was held on Wednesday.

His endorsement would be the biggest Obama has secured so far. As a prominent member of President George Bush's administration, he may shift some Republican-leaning voters into Obama's camp.

McCain's campaign team admitted privately it would be a blow, not least because it would be another day in which Obama dominated the news. But Democratic strategists are divide about the benefits. One said yesterday he was concerned it could backfire. He suggested that Powell's decision might be seen as being based on racial solidarity and that could alienate some white voters already reluctant to back Obama on grounds of race.

The sceptical strategist added that Powell, though once so popular he was considered as a potential candidate for the White House, was badly tarnished by his involvement, as secretary of state, in making the case for Saddam Hussein having weapons of mass destruction.

Powell, 71, who is to appear on NBC's Meet the Press, told a conference in Washington last month that he was undecided. "I've said to my beloved friend and colleague, John McCain, a friend of 25 years - John, I love you - but I'm not just going to vote for you on the basis of our affection and friendship," Powell said.

"I've said to Barack Obama, I admire you, I'll give you all the advice I can, but I won't vote for you just because you're black. We have to move beyond this."

Obama won the endorsement yesterday of the Washington Post. "There are few public figures we have respected more over the years than Senator John McCain. Yet it is without ambivalence that we endorse Senator Barack Obama for president," the Post said.

The newspaper said McCain had lost its support because of the way he conducted his campaign - singling out his choice of Sarah Palin as his running mate. The Post was the 40th newspaper to endorse Obama for president. So far McCain has the support of 15 newspapers.

The presidential race continued to be transfixed by Joe the Plumber, the Ohio man invoked by McCain in this week's final debate as the symbol of what he claimed was Obama's "class warfare" against ordinary working people. Joe Wurzelbacher's 15 minutes of fame is now stretching into its third full day and shows no sign of abating.

McCain organisers have been trying to get Wurzelbacher to appear alongside McCain at a campaign rally in Toledo, Ohio, tomorrow. But invitations to appear on the weekend's TV talkshows may prove more alluring. Though he has refused to say who will get his vote, Wurzelbacher's story has prompted a fiercely partisan reaction from the US media.

Liberal bloggers focused on idiosyncracies in his personal details - the fact that he owes almost $1,200 in back taxes, that he has had no formal plumbing training and holds no licence, and that his full name is not Joe at all but Samuel J Wurzelbacher.

Rightwing bloggers retaliated against what they called such smear-mongering. "The left's political plumbers are attacking the messenger ... It's standard operating procedure for the Obama thug machine," wrote Michelle Malkin.

In Pennsylvania on Thursday McCain told a rally that Wurzelbacher had been the real winner of the debate "because people are not going to let Senator Obama raise taxes in a tough economy", prompting chants of "Joe! Joe! Joe!" from the crowd.

In New Hampshire, Obama asked supporters: "How many plumbers do you know making a quarter of a million dollars a year?"

Meanwhile McCain's running mate, Sarah Palin, is taking her critics at first hand by appearing in Saturday Night Live. The entertainment show has been running a popularly parody of the Alaska governor by comedian Tina Fey.

The week

Confession of the week

"If I were watching Fox News, I wouldn't vote for me, right? Because the way I'm portrayed 24/7 is as a freak! I am the latte-sipping, New York Times-reading, Volvo-driving, no-gun-owning, effete, politically correct, arrogant liberal. Who wants somebody like that?" - Barack Obama, quoted in the New York Times

Unsolicited phone call of the week

Ring. Ring. Ring. "Hello. I'm calling for John McCain and the RNC. You need to know that Barack Obama has worked closely with domestic terrorist Bill Ayers, whose organisation bombed the US Capitol, the Pentagon, a judge's home, and killed Americans." Click . Brrrrr .

Geography lesson of the week

"It seems like, and in our last rally too, and in other parts around this great Northwest, here in New Hampshire, ya just get it." - Sarah Palin, on the trail in the north-eastern Granite State.

Aspiration of the week

"I'm not Matt Damon or one of those guys who has droves of men and women wanting to be them." - Joe the Plumber, assessing the potential impact of his new-found celebrity, which has spawned Joe the Plumber lookalikes at rallies

Gag of the week:

"I do love the Waldorf Astoria. I hear that from the doorstep you can see all the way to the Russian Tearoom." - Barack Obama