The Idaho Constitution says that counties may be divided among legislative districts only to the extent necessary to comply with the U.S. Constitution, whose one-person, one-vote rule requires that districts be apportioned by population; a population deviation between legislative districts of more than 10 percent is presumed to be unconstitutional.

Idaho's Supreme Court decision today overturning the state's new legislative district plan said it violated that clause in Idaho's Constitution, in Article III, Section 5, by dividing too many counties.

Justice Jim Jones, in a 14-page dissent, differed with the majority, writing, "If this Court imposes a strict requirement that the Commission adopt whatever plan meets the ten percent population deviation and produces the lowest possible number of county splits, the Commission's jurisdiction will be limited to the point that it will have no realistic function."

But the majority, in an opinion written by Justice Daniel Eismann, wrote, "Plan L-87 divides 12 counties. The commission considered and rejected other plans that comply with the federal constitution and divide fewer counties. Thus, Plan L-87 does not divide counties only to the extent that counties must be divided to comply with the federal constitution." He also wrote, "If one plan that complies with the federal constitution divides eight counties and another that also complies divides nine counties, then the extent that counties must be divided in order to comply with the federal constitution is only eight counties. It could not be said that dividing one more county was necessary to comply with the Constitution."