Barack Obama should be 15 points ahead of John McCain in the opinion polls at this stage of the campaign yet they are still neck and neck, Edward Rendell, the governor of Pennsylvania and a prominent supporter of Hillary Clinton’s former presidential bid, said on Tuesday.

Mr Rendell, who was speaking to the Financial Times just hours before Mrs Clinton delivered her prime-time plea for the Democratic party to unite behind Mr Obama in Denver on Tuesday night, said the Democratic nominee was still struggling to get his message through to blue-collar voters in states like Pennsylvania.