Political advocate: Campaigns for elective office should be subsidized with public funds.
Advocate's Conclusion
We should subsidize campaigns with public funds.
Advocate's Evidence
It would save incumbents the time they spend fund-raising and, if it allowed us to set caps on max contributions, it would reduce the amount that candidates are beholden to large contributors, both of which should mean that the politician would be better able to serve the public.
Critic's Objection
If you set a cap on contribution size, then candidates will end up spending more time seeking our small contributions.
Evaluate
The critic is essentially saying that one unintended (and undesirable) effect of the advocate's plan would potentially screw up one of the intended desirable effects.
The advocate is hoping that by providing subsidies (i.e. by giving candidates $100k of public money to run a campaign), an incumbent politician would suddenly have more time on their hands to serve their constituents -- they wouldn't be worried about trying to raise $100k of contributions in order to fund their re-election campaign.
But, the advocate also said that by giving candidates a subsidy we could set a cap on individual campaign contributions. Maybe right now individuals can contribute up to $10k, whereas we could set a cap saying you can contribute at most $1k.
In the current system, maybe a candidate raises $100k by relying on ten big donors who each contribute the max. As a result of their big donations, the candidate feels obliged to look out for their interests, to "work for their benefit".
The critic's objection is that if we set contribution caps, then now a candidate would need to find 100 donors to contribute $1k each instead of 10 donors to contribute $10k. Since that is ten times as much work soliciting campaign contributions, it means an increase in the time-drain of fund raising.
Goal
So the critic is saying, "one of your suggestions your plan makes would have a ramification that would undermine one of the intended benefits of your plan.