Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I say who wrote them.
Denzau and Munger. 1986. Legislators and interest groups: How unorganized interests get represented. APSR (1986): 86-106.
Derives a supply price for public policy. Shows that interest groups will generally target legislators whose voters are indifferent to the policy the interest group seeks. Thus, even unorganized voters can have their preferences represented.
Rational interest groups will channel their money towards the swing votes on a committee (i.e. those with a supply price closest to zero). It would be a wasted to try buying off legislators with a high supply price, since opposing interest groups could give the same legislators a very small sum and thereby keep these high-price legislators on their side. By the same logic, interest groups wouldn't try to buy off legislators that already support them (i.e. a negative supply price), unless they needed to counter an attempt by opposing interest groups. Thus, all groups will try to outbid one another to buy off the legislators with the supply price closest to zero; these are the swing voters on each committee.
Since there interest groups from both sides target the same swing voters, the relatively indifferent legislators might receive lots of campaign resources (see p 98).
If voters are rationally ignorant, you might think that interest groups completely control the legislator's activities (subject only to the constraints of 'E'). This is mistaken, however. Even if the voters are ignorant, self-interested challengers and the media have incentives to discover and advertise the incumbent's wayward behavior. Thus, even rationally ignorant voters have their preferences represented.
If voters are perfectly informed, then interest groups can't buy off any legislators. If they get what they want from legislators, it's only because the legislator's constituents support it.
Interest groups will seek out those legislators who have generally indifferent (about a given policy) constituencies. They can thus have some influence on legislative outcomes, though this influence is constrained.
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