I don't agree that the Liberal brand is holding that party ahead of the NDP. This is a myth. I think the NDP brand increasingly has a value- one that is not marked by scandal in the way the Liberals are.
I think the Liberals remain ahead because we have not won the idealogical arguments our policies rest on with enough people.
This is one reason why catering to the right does not work for the NDP- the party has to be consistant and work to win people over time on the idealogical questions about power, conflict, the role of government, equity, human rights etc. When these are won you will see just how weak the Liberal brand is.
I do not hear of people voting Liberal because of the brand-- rather I hear people voting Liberal while holding their noses because they think it is safer or middle of the road or moderate etc. But people are not in love with that party-- at least no Liberal voter I know. Of course my sample size declines year over year-- I work on some and others just gradually get with the tour on their own...