Japan’s prime minister, Shinzo Abe, has called for a debate on rewriting the country’s pacifist constitution after his Liberal Democratic party [LDP] and its allies secured a supermajority in upper house elections on Sunday.

The LDP, its junior coalition partner Kōmeitō, and several like-minded smaller parties and independent MPs now control two-thirds of the 242 seats in the upper house. The ruling coalition already has a similar majority in the more powerful lower house.

Conservative MPs have enough seats to push ahead with constitutional changes, including scrapping the war-renouncing article 9 – a prospect that has caused alarm in China and among many Japanese who value their country’s postwar pacifism. Any amendments passed in parliament would then require approval by a simple majority in a nationwide referendum.

Abe had studiously ignored the constitution issue during the upper house campaign, insisting that the election was an opportunity to reaffirm public support for his troubled economic policy, as he sought to capitalise on the lack of a credible alternative offered by the opposition.

The LDP won 56 of the 121 seats – half the upper house total – being contested, while Kōmeitō secured 14 seats. Abe had set a goal of winning a combined 61 seats.

But speaking soon after his landslide victory, Abe said his party had always been committed to rewriting the postwar constitution. The Asahi Shimbun newspaper quoted him as saying that he hoped deliberations by expert panels and a deeper public debate would lead to a consensus on which parts of the constitution needed changing.

The most controversial move would be a revision of article 9 to allow Japan’s self-defence forces to act more like a conventional army. The clause forbids Japan from using force to settle international disputes and restricts its land, air and naval forces to a strictly defensive role.

Rewriting the constitution, imposed by the US occupation authorities after the second world war, has been the ideological driving force behind Abe and other conservatives who believe it unfairly restricts Japan’s ability to respond to new threats such as international terrorism, an increasingly assertive China and a nuclear-armed North Korea.

However, Abe risks losing the political capital he has built over the past three and a half years if he is seen to be neglecting the economy in favour of constitutional reform.

“The key question will be whether he can carry out structural reforms,” said Nobuhiko Kuramochi, chief strategist at Mizuho Securities. “If Abe fails to do so, despite the political freedom he has gained, that will be negative for foreign investors’ appetite for Japanese stocks.”

Xinhua, China’s official news agency, described Sunday’s election result as a threat to regional stability, as it had given MPs who support constitutional reform an unprecedented advantage.

“With Japan’s pacifist constitution at serious stake and Abe’s power expanding, it is alarming both for Japan’s Asian neighbours, as well as for Japan itself, as Japan’s militarisation will serve to benefit neither side,” Xinhua said in a commentary.

Some analysts played down the prospects for change, noting that the loose collection of pro-reform parties and independents had yet to reach a consensus on which parts of the constitution should be altered.

“It’s the first time to have two-thirds in both houses of parliament, but you can’t find any issue on which the two-thirds can agree,” said Gerry Curtis, professor emeritus at Columbia University.

But Curtis added: “With these numbers … he [Abe] is going to want to see what he can achieve. That means less attention to the economy and a lot of spinning over the constitution.”

The LDP’s dovish coalition partner, Kōmeitō, is cautious about any change that would expand the role of the military, while the public remains deeply divided. An exit poll conducted by the Asahi on Sunday showed that 49% of voters supported constitutional revision, with 44% opposed.

