We study two-party elections considering that: a) prior to the voting stage voters are free to trade votes for money according to the rules of the Shapley-Shubik strategic market games; and b) voters' preferences --both ordinal rankings and cardinal intensities-- are public information. While under plurality rule no trade occurs, under a power-sharing system (voters' utilities are proportionally increasing in the vote share of their favorite party) full-trade is always an equilibrium (two voters --the strongest supporter of each party-- buy the votes of all others). Notably, this equilibrium implements proportional justice with respect to the two buyers: the ratio of the parties' vote shares is equal to the ratio of the preference intensities of the two most opposing voters.