Japanese Prime Minister Shinzo Abe. (Koji Sasahara/Associated Press)

AGREE OR disagree with his policies, there is no denying that Japanese Prime Minister Shinzo Abe has been a political success. Sunday night he led the Liberal Democratic Party to its second decisive victory in a national election since the one in December 2012 that propelled Mr. Abe to office after four years of rule by the LDP’s opponents, the Democratic Party. Though sometimes unsettling to Japan’s neighbors, Mr. Abe’s decisive, more nationalistic tone clearly pleases his country’s voters. And, in the main, his approach to the multiple problems of his strong but stagnant country has warranted the qualified endorsement it has received from the Obama administration.

As of Sunday night, Mr. Abe for the first time has the supermajority in both houses of the Japanese parliament that it would take to initiate constitutional changes granting wider latitude to Japan’s military, which has been constrained — unduly so in Mr. Abe’s long-held view — by pacifist provisions of the postwar document. Though polls suggest those changes would be less popular than Mr. Abe’s government itself, he is not wrong to worry about Japan’s security posture given the assertive attitudes in Beijing and Moscow as well as the U.S. public’s seeming turn away from international engagement.

Whether an attempt to alter public opinion on constitutional change would be a wise use of Mr. Abe’s large political capital is another question, however. Once approved by Japan’s parliament, any amendment would still need to win a national referendum, after a likely bitter campaign that could divide the country and absorb the prime minister’s energy — for a cause of unclear necessity. Mr. Abe has already won legislative approval for joint military missions with the United States and other allies, including missions not related to Japan’s own territorial defense.

Better for Mr. Abe to focus on refloating the dead-in-the-water Japanese economy, still plagued by deflation, negligible growth and declining exports more than three years since the ballyhooed launch of “Abenomics.” Massive injections of stimulus, both fiscal and monetary, have essentially failed; yet the “third arrow” of Abenomics — structural reform to labor markets, corporate governance and agriculture — has barely been implemented. Given fierce resistance from entrenched interest groups, this was always bound to be Mr. Abe’s most difficult task. And American presidential nominees are not making it easier for him by denouncing the Trans-Pacific Partnership trade agreement, which helps lay the basis, economic and political, for further Japanese reform.

Nevertheless, Mr. Abe should make a genuine economic shake-up his top priority, ahead of his constitutional project. Either one is bound to be politically costly, but economic reform offers the prospect of restored prosperity, without which Japan won’t be able to mount a more robust national defense no matter what the constitution says. The battle for a new Japanese economic model is the right fight to pick.