Barack Obama is recruiting senior staff to a new unit which will combat virulent rumour campaigns on the internet that threaten to cost him votes in the presidential election against John McCain.

The unit is part of a huge expansion of Obama's campaign team as he shifts from the Democratic nomination race to the campaign for November's election.

As well as the rumour-mongering problem, units are being set up to deal with other perceived vulnerable points, including off-the-cuff remarks by his wife Michelle. McCain's wife, Cindy, questioned Michelle's patriotism in February after she said: "For the first time in my adult life, I am proud of my country."

Brooks Jackson, director of the Washington-based FactCheck.org, an independent academic organisation set up in 2003 to monitor the factual accuracy of statements made in elections, said yesterday there had been false rumours on the internet about George Bush and John Kerry in the 2004 election.

"With Obama, it is particularly vicious," Jackson said. He added that one of the most persistent is that Obama, a Christian, is "some kind of Muslim Manchurian candidate, planted by Islamic fundamentalists to betray the country and it is very widespread".

McCain too suffers from rumours on the internet, mainly from former Vietnam veterans disputing his account of his five-and-a-half years in a Hanoi prisoner-of-war camp.

Obama yesterday began a three-week campaign through potential swing states in which he will focus on concerns about job losses, collapsing house prices and soaring petrol costs. McCain, short of funds compared with Obama, held fund-raising events.

The latest Gallup poll nationwide puts Obama on 46% against McCain's 44%. CBS has Obama on 48% and McCain on 42%. Obama is ahead in the key swing states of Ohio and Pennsylvania, while McCain leads in Florida. In Virginia, normally a state Republicans can rely on and Obama is targeting, McCain is only 1% ahead.

Obama delivered his speech in Raleigh, North Carolina, which he has only an outside chance of taking, but his campaign team said he wants to send a message he is going to compete in all 50 states, not just the swing ones.

Obama paid homage to McCain as an American war hero and acknowledged that the two shared many of the same goals such as tackling climate change.

But Obama said the two had fundamentally different views of how to deal with the economy. Reflecting Obama's strategy of tying McCain to the unpopular Bush, Obama said: "For all his talk of independence, the centrepiece of his economic plan amounts to a full-throated endorsement of George Bush's policies."