This is going to be a but long-winded so bear with me, but in essence whether the data is public or not would appear to be irrelevant (as always IANAL)
The second data protection principle states that:
Personal data shall be processed fairly and lawfully and, in particular, shall not be processed unless –
(a) at least one of the conditions in Schedule 2 is met, and
(b) in the case of sensitive personal data, at least one of the conditions in Schedule 3 is also met.
Since it's public we can assume only (a) will apply here (personal data doesn't stop being personal data just by being published). Now on to schedule 2 of the DPA, which states:
1 The data subject has given his consent to the processing.
2 The processing is necessary—
(a)for the performance of a contract to which the data subject is a party, or
(b)for the taking of steps at the request of the data subject with a view to entering into a contract.
3 The processing is necessary for compliance with any legal obligation to which the data controller is subject, other than an obligation imposed by contract.
4 The processing is necessary in order to protect the vital interests of the data subject.
5 The processing is necessary—
(a)for the administration of justice,
[F1(aa)for the exercise of any functions of either House of Parliament,]
(b)for the exercise of any functions conferred on any person by or under any enactment,
(c)for the exercise of any functions of the Crown, a Minister of the Crown or a government department, or
(d)for the exercise of any other functions of a public nature exercised in the public interest by any person.
None of which appears to apply to what the Labour party have been doing (unless they truly think that anything is acceptable in the name of politics, and that public interest is the same as party political interest - or that they're confusing 'data subject' with themselves when it comes to 'vital interests'). And of course given previous behaviour by the likes of the Labour party in other cases I would also ask whether adherence to the 7th and 8th principles have been observed to in this case.