Voters have a eight-year tolerance for prime ministers. Any longer than that, and they get restive and fretful. We saw that in the run-up to the 2005 election, where large amounts of left-of-centre people were fed-up with Blair and his zeal to get us involved in wars that had nothing to do with us.
Luckily for Labour, Blair announced unprompted in late 2004 that he was standing down in the next parliament. So voters knew they'd be getting a new Prime Minister. And Labour was careful to show Blair and Brown together throughout the 2005 campaign. We were as open as possible with the voters in letting them know who would succeed Blair. So the voters knew they were getting a change - the question was which change they wanted - a change to Brown or a change to Howard. They chose Brown and Labour.
Roll on to 2010, and the Tories are campaigning on Vote for a Change, and have been careful to let us know that there will be Swinging Austerity ahead if they win. In other words, if you vote Tory, you will be voting for A Change for the Worse. Only voters don't want any change at all, let along a change for the worse. What they want is things to go back to how they were in 2007. Given that 2007 was Gord's glad morning, large amounts of people think he's the one who knows how to restore that moment - hence the tightening in the polls.