The Supreme Court ruling that disparity in the value of votes cast in the 2013 Upper House election was "in a state of unconstitutionality" — though it once again fell short of invalidating the election result itself — should serve as a stern rebuke on Diet members and their parties that continue to drag their feet in overhauling the electoral system even as time runs short before their self-imposed deadline for fixing the problem.

In the July 2013 poll that gave control of the upper chamber to the ruling coalition led by Prime Minister Shinzo Abe, the maximum gap in the value of votes between electoral districts reached 4.77 to 1. It means that people in the constituency with the smallest number of eligible voters per Upper House seat allocated to the district had 4.77 times more power in electing lawmakers than those in the constituency with the largest number of voters per seat. Such a disparity runs counter to the principle of equality under the Constitution and distorts the representation of popular will in the Diet.

When the Upper House election system was introduced in 1947, the maximum disparity in the vote value was 2.62 to 1, but the gap has since widened with the postwar population shift from rural to urban areas. It had reached as wide as 6.59-to-1 in the 1992 election, which four years later the Supreme Court determined for the first time was in a state of unconstitutionality — an expression that fell short of judging the election as unconstitutional and thereby illegal.

After the gap was narrowed with reallocation of seats between constituencies, the top court sanctioned a disparity of around 5 to 1 in the triennial elections held between 1995 and 2007 as constitutional. On the 2010 election, however, the Supreme Court ruled again in 2012 that the gap of 5 to 1 was in a state of unconstitutionality — an indication that it is taking a harder look at the huge inequality in the value of the votes.

The Diet responded to the 2012 ruling by revising the election law to cut four seats from less populous constituencies and add as many seats to more populous districts, thereby trimming the disparity to 4.77 to 1 in the election held in July 2013. Still, the top court decision on Wednesday again dismissed the minimum reapportionment of seats as not enough to bring the gap in the value of votes within constitutionally acceptable levels. The ruling makes it clear that more such minor adjustments in the electoral system will not resolve the problem.

The Supreme Court ruling in fact marks a step back from some of the 16 high court decisions handed down last year on the July 2013 election — three courts said the vote value gap was an outright violation of the Constitution, including the Okayama branch of the Hiroshima High Court, which ruled the election result was invalid. Among the 15 justices on the top court's grand bench who gave the ruling on Wednesday, four opined that the 2013 election was unconstitutional and one of them noted that the vote result should be invalidated.

The top court dismissed the lawyers' plea for invalidating the election outcome, saying that the vote value gap cannot be ruled outright as unconstitutional given the limited period of time available for the Diet to introduce fundamental reforms after the October 2012 ruling in time for the July 2013 poll. In the 2012 ruling, the top court called for legislative steps to overhaul the Upper House electoral system, including changing the current electoral districts, in which each of the nation's 47 prefectures serves as an electoral district. But it was not the first such warning by the top court. Its 2009 ruling on the 2007 election — which judged the vote value gap itself was constitutional — also called for a review of the Upper House electoral system. Still, no such action has since been taken by the legislature. One of the justices who said the gap in the 2013 race was unconstitutional noted in his opinion that the Diet had more than three years since the 2009 ruling to act on the electoral system overhaul.

Members of the Diet must not take the top court's lenient decision as giving them a grace period for inaction. The 2012 revision to the election law called for a fundamental reform of the electoral system by the time the next Upper House election is held in 2016. Given the time that would be needed for publicly notifying the changes well ahead of the race, time is running short before they can implement the overhaul of the system for the 2016 election. However, the parties and their lawmakers involved in the discussion on the electoral system reform remain divided.

Reducing the number of seats allocated to less populous constituencies is sure to face opposition from the lawmakers elected from those districts, who typically argue that such reforms would leave the voices of people in rural areas unheard in national politics. It's not surprising that the biggest resistance comes from the party that has won the largest number of seats in such constituencies in recent elections.

In April, Masashi Waki, then secretary general of the LDP's Upper House caucus who headed the chamber's council on electoral reforms, presented a draft reform that would combine 22 prefecture-based constituencies into 11 new districts, thereby narrowing the maximum gap in the value of votes to 1.83 to 1. Under the current system in which half the chamber's seats come up for election every three years, each constituency has to have at least two seats, and there are limits to substantially narrowing the gap in vote value without redrawing the electoral districts across prefectural borders.

Waki's proposal predictably met with instant rejection from within his LDP colleagues, and was effectively withdrawn when he was sacked from the position in September. The LDP has since floated various ideas that would avoid radically redrawing the constituencies and retain a wide gap in the value of votes, and the prospect remains dim for a consensus with other parties.

The participants in the discussions need to realize that the issue at stake concerns the foundation of parliamentary democracy. As one of the top court justices said, disparity in the value of votes between constituencies creates a regional imbalance in political power, and the constitutional principle that the nation's sovereign power rests with people will prove a pie in the sky unless their equal voting rights are guaranteed. The parties and lawmakers need to set aside their own interests to quickly resolve this problem. If they fail to do so, they will leave the judiciary with little choice but to compel them to take action.