Edit: You have two ethical ways
- Be a leader yourself. You don't have to come to or try to come to power for that. You only have to lead people in thought.
Influence them enough and you can have them protest. This is very effective because it don't require a large percentage of voters, it also give results in shorter than come-in-power time.
With enough influence you can use people's donations to party funds as a tool too.
Ofcourse you can start your own party as well. This ofcourse means you have to become a leader to-come-in-power, not just an influencer.
- Go to court.
Its always in your power to go to court. Its not in your power to have a court decision in your favour. Heck, court can even refuse to give you a hearing. May be court is biased or there genuinely is no law in your favour to base your case on.
Still you can try knocking that door. Nobody can blame you later when you take extreme step (see below) if you did use this option.
- Start a revolution.
Ofcourse the assumption here is that your motive - Influence A Single Issue - is ethical, as in other options given above.
The Single Issue has to be serious enough, obviously, to warrant an entire revolution.
For completion I will also list three non-ethical options too.
- "Lobby" (bribe) [Non-Ethical]
Its non-ethical because it goes behind the backs of populace and make deal with a politician directly without consent of populace.
You ofcourse have to be rich to go on that path, and non-ethical.
- "Lobby" Through Media [Non-ethical]
Have some editorials and other articles written in your favour; newspapers, tv, websites ads etc; minor news emphasized; that show larger than reality picture of the issue.
Ofcourse you have to be rich for this and unethical.
- Spread Rumours [Non-ethical]
Takes money, and population that don't confirm before spreading.
Note: There can be ethical lobbying. For example if you can influence / control votes of a minority you can use that as a bargaining chip in an expected tight race between two candidates. Some lobbying is bad, I put it in quotes because its not how lobbying shall be.
1.In short of becoming a leader yourself, no, you do not have an ethical way.
You can use money, if you are rich, for "lobbying" (really just bribing). And that's it.
If you decide to go down the leadership path, you can start convincing other people. Once convinced, they can donate money. You can use that for "lobbying". You can also use them as a pressure group by protesting, using them as a bargaining chip in "lobbying", etc.
- You can always go to court.