The Ninth Circuit has refused to rehear en banc its earlier decision upholding a First Amendment right to create a vote swap site.
A legislator promising to vote a particular way if another legislator votes a particular way: ordinary log-rolling. [Trading votes for votes in the legislature is thus seen as vastly different from trading votes for money.]
A legislator promising to vote a particular way if voters elect him: ordinary and constitutionally protected (Brown v. Hartlage) campaign promises.
Voters promising to vote for a legislator if the legislator promises to vote a particular way: the example [given here], which I think is quite proper.
Voters promising to vote a particular way if other voters promise to vote a particular way: that's voteswap.com, and it seems to me hard to see why it should be a crime when the others are permissible and even constitutionally protected. If legislator-legislator, legislator-voter, and voter-legislator deals are permitted, why not voter-voter deals?