How it went wrong
I have a long post up on PoliticsHome looking at how Labour got into the position of being 20 points behind, based on the daily data produced by their Phi5000 panel.
It's very clear that the point when their reputation really crumbled was during the 10p tax row, when approval of the government collapsed, they were seen as increasingly divided and the last few shreds of Gordon Brown's reputation for competence fell away. In contrast, the Conservative increase then is still mostly just a result of them looking good when compared to Labour - their negatives have reduced, but there is no obvious positive boost for them. The exception is Cameron himself, whose reputation has improved significantly since the local elections and mayoral elections, particularly in terms of competence and effeciency.
Still, that is how we got here. If you look at the graphs over on the post you can also see a couple of wobbles in the Conservative ratings over the last week or so, look particularly at the way the percentage of panellists who think the Conservatives are united has dropped, and those who aren't clear what they stand for has increased since David Davis's resignation.