When you intend something, you can in some circumstances be reasonably regarded as having a different intention, or a further intention.
The primary intention of your campaign may not be to influence voters. For example, you might run a campaign with one or more of the following intentions:
raising awareness of an issue
influencing political parties to adopt a policy in their manifestos
campaigning for or against government legislation
providing information to voters
encouraging people to register to vote
encouraging people to vote, but not for anyone in particular
A campaign that can be reasonably regarded as having one of these intentions will not meet the purpose test unless it can also be reasonably regarded as having the intention to influence voters to vote for or against a political party or category of candidates.
Even if your primary intention is something else, your campaign will still meet the purpose test if it can be reasonably regarded as intended to influence voters to vote for or against a political party or category of candidates.
For example, suppose for example your intention is to influence political parties to adopt a policy. If you go about this by identifying and promoting parties and candidates who have already adopted the policy, then this will meet the purpose test.