In the sentence below, the position of the word "once" does not seem right. The phrase sounds archaic to my ears. Is it correct, or is "than it did once" or "than it once did" better?
The Party's claim to now be Labour's main challenger in the North of England seems far more convincing than once it did.
It's not wrong, though.
It's unusual but not wrong. You would see "than it once did" more often.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
I think the writer may be attempting a flowery flourish with the word order to give the change a greater emphasis.