Not at all -- tragedy-of-the-commons situations depend on freeloaders being able use that freeloading to further amplify their ability to freeload -- e.g. the guy that overgrazes the field produces more sheep than the restrainers, leading to further overgrazing (until collapse).
Under the assumption (again, with non-obvious implications) that voters intelligently reject bad ideas within the marketplace thereof, there is no such self-reinforcement -- they reject the bad ideas in favor of better ones. It would be like a magical commons that poisons any sheep whenever the blade of grass it eats is beyond its quota. Or, more realistically, like a commons whose polity has set up rules with punishments for those who defect from the solution to such a collective action problem.
>(Also, rational agents -- by the definition of rationality -- don't act on the social consequences of options, but on their personal costs and benefits.)
That's not the definition of rationality, but of rational agents with a particular set of preferences (i.e. of caring only about themselves). You can be rational and still care about starving kids in Africa.
But irrespective of semantic games, the relevant distinction for this issue is whether the voters are rational in the sense of identifying and rejecting bad ideas. To the extent you claim they meet this definition then (irrespective of how much altruism you want to insist is "rational"), the destructive propaganda of the wealthy would be stopped dead in its tracks by intelligent voters, rendering campaign finance moot -- but that obviously is not true, so it's hard to come up with a model that has it both ways.
(Of course, FWIW, the intelligent-but-dont-call-it-rational voter has to have some altruism in order to vote in the first place.)
(1) There is no such thing as a "bad idea". There are ideas that have net negative utility for particular voters. But these same ideas may have net positive utility for other voters.
(2) Campaign spending doesn't have to get people for whom an idea is bad (net negative utility) to accept it to be useful to propagate, as long as it does either of the following (a) reach people to whom it is of net positive utility, who will adopt it and act on it politically, or drive up the price of the resources (media access, etc.) needed to spread competing ideas that are net negative utility to the actor deciding to spend resources on the current idea, so as to reduce the cost to that actor from the uptake of those harmful ideas.
Of course, with rationality (which, aside from personal utility maximization also includes perfect information about all alternatives including their costs and benefits), propagating ideas (political or otherwise) would be unnecessary, since every one would know and choose their best alternative without needing to be exposed to or propagate ideas. But if you weaken the assumptions of rationality only enough to make propagating ideas meaningful (that is, abandon knowledge of all alternatives, but still assume that once exposed to a proposed alternative people can perfectly weigh its utilities against all other alternatives they have been exposed to), then greater inequality in access to resources to propagate information still has a distorting effect without further assuming people ever make any errors in assessing the relative merits of ideas that they are exposed to, and there is still a role for campaign finance restrictions.
It is simply false to say that you need a model in which people cannot effectively process information they receive about policy alternatives to justify campaign finance limits: it can be justified while assuming people are perfect evaluators of alternatives they are exposed to.
You can't justify it if you assume people are perfect evaluators of their alternatives without a need to be exposed to them, but with that assumption there is literally never any utility to campaign spending (and, as it has a cost and no utility, no one would do it if they were perfect evaluators of their alternatives), so I think we can reject that assumption as being useful in any universe in which campaign spending (limited or not) actually occurs.
That is a huge assumption that doesn't seem to match the world we live in. Your best attempt to justify it is with a claim that voters, while not morons, make a small amount of errors that is somehow enough to fail to appreciate the tidal wave of "let's not get exploited" advice they hear from friends, who would want to spread it like the dickens.
I don't see how that constitutes a reasonable model. At some point, you have to blame the voters themselves.
[1] And I'm going to assume you know here that I mean "bad idea" in the sense of "the purported idea that goes contrary to most voters but gets propped up by money" so as to save me the talk about "there are only bad ideas for particular voters".
Actually, expending effort to propagate even subjectively good political ideas (that is, ideas that would benefit the person involved if they were widely held) is usually irrational for most people, because the expected marginal utility is low compared to the cost to spread them.
Its useful to the individual if other people spread the ideas, but its often a classic tragedy of the common situation.
Now, for wealthy people (or people with otherwise disproportionate media reach and impact), the utility cost of propagating useful ideas to those who might be receptive is lower compared to the benefit, so its more likely to be rational to do it. Further, with unconstrained spending, the spending of the rich bids up the market cost of reach for ideas, making it less rational for the less rich to expend resources propagating the ideas they favor.
You are invoking rationality, but doing so with an argument that ignores market conditions, opportunity costs, and relative payoffs.
(Also, rational agents -- by the definition of rationality -- don't act on the social consequences of options, but on their personal costs and benefits.)