Which is correct: "lying gang of scum", or "gang of lying scum"?

These are both correct, but the two structures have different meanings. However, in your particular example, with the adjective lying, there is effectively no difference.

Imagine if you replace lying with the word enormous.

• Enormous gang of scum
• Gang of enormous scum

Either the gang as a whole is enormous or the individual members are enormous.

The difference with lying is that a lying gang is automatically interpreted as one where its individual members lie, because lying is something only individuals can do. So a gang of lying scum is effectively the same thing as a lying gang of scum.

Either is more or less correct in my view, though I would probably opt for the latter (lying scum), since it is more accurately the "scum" to whom the description "lying" is applied. "Gang" is more of an abstract concept - a group of people - you're less likely to attribute laziness to that.

I think they're both correct, but with possibly slightly different meanings. I don't think Kosmonaut is right to say that "lying is something only individuals can do". A gang can collectively tell a lie, especially if that gang is a political party. 'Manifesto commitments' could very well be lies.

So perhaps if you're more interested in emphasising the tendancy of individual politicians to tell lies then you should call the party a gang of lying scum, but if you want to emphasise the tendancy of the party as a whole to tell lies then it is a lying gang of scum.