Japan’s Prime Minister Shinzo Abe speaks from the roof of a campaign van in Okinawa city on July 16. Polls suggest that Sunday’s parliamentary election will provide an overwhelming victory for his ruling Liberal Democratic Party. (HITOSHI MAESHIRO/EUROPEAN PRESSPHOTO AGENCY)

After cycling through seven leaders in seven years — many who squandered their support months, if not weeks, into their terms — Japan is about to break free from its most vexing political pattern. It’s on the verge of stability.

Polls suggest that Sunday’s parliamentary election will provide an overwhelming victory to the ruling Liberal Democratic Party (LDP), giving Prime Minister Shinzo Abe a chance to become Japan’s most transformative leader since the collapse of the bubble economy two decades ago.

An election in December in the more powerful lower house brought the LDP and Abe into power. But this election — for 121 of the 242 seats in the upper house — will give the party a chance to claim majorities in both chambers, breaking a long-standing “twisted Diet” system that slowed down or stopped even modest policy proposals.

Analysts say that Abe, once his party is firmly in power, will have a mandate unmatched by anyone since the popular Junichiro Koizumi, who left office in 2006. But Abe’s ambitions are greater, and his challenges are bigger, those analysts say. Japan is locked in a territorial struggle with China, sapped by energy shortages and weighted down by a massive government debt.

For policymakers in Washington, as well as those in neighboring Asian countries, Abe’s increased power is welcome so long as he focuses mainly on reviving the economy. But Abe, whose grandfather was arrested but never charged as a war criminal, has deeply rooted nationalist feelings, and he has a track record of downplaying Japan’s World War II atrocities. In the past months, Abe has kept his personal views largely bottled up. But some opposition lawmakers in Japan fear an emboldened Abe could speak more openly about his revisionist beliefs, enraging China and South Korea and aggravating the United States, which is pressing Tokyo to play nice with its neighbors.

In his seven months in power, Abe has tried foremost to re-energize Japan’s long-stagnant economy, doling out stimulus funds while successfully pressuring the central bank to loosen its purse strings. Those tools have helped drive up the Nikkei stock index about 60 percent, while leaving Tokyo’s major corporations flush with profits. But economists and government officials in Tokyo say the revival will be sustainable only if Abe can carry through with broad economic reforms, including difficult changes in agriculture, the social security system and the medical industry.

Abe has already unveiled one starter plate of ideas, but markets were underwhelmed. “Some gutsier initiatives will probably be required to think, ‘Aha, he’s really serious about this reform,’ ” said Sheila Smith, senior fellow for Japan studies at the Council on Foreign Relations.

Political resurrection

Abe, until recently, seemed among the most unlikely candidates to provide Japan with stability.

He washed out in his first stint as prime minister, a woeful one-year term in which he pursued a string of nationalist pet projects that polls suggest the country had little appetite for. Abe eventually resigned because of a bowel ailment.

In 2007, in the middle of Abe’s first term, the LDP lost a majority of its seats in an upper-house election, a setback from which the party only now could fully recover. With Japan’s two parliamentary houses dominated by different parties, a series of prime ministers struggled to gain support or pass legislation. Just as important, the frequent replacement of foreign and defense ministers short-circuited Japan’s efforts to build ties with its neighbors and its traditional ally, the United States.

“That defeat [in 2007] was the start of it all,” Abe said in a news conference late last month. “[Japan’s] politics has been wandering, with the prime minister changing to one person and then another year after year — myself included — and Japan’s national strength came to wane substantially.”

After Sunday, it might be three years before Japan holds another parliamentary election, and Abe might go just as long without a challenge to his leadership. Still, he will not have carte blanche.

Military moves

One of Abe’s fundamental goals is to turn Japan into what he calls an “ordinary country” — one with a standing military that has purposes beyond self-defense. But to loosen most of the major restrictions on Japan’s Self-Defense Forces, Abe must revise the pacifist constitution — a contentious move in a nation with powerful memories of its past militancy. New Komeito, the LDP coalition party, wants the constitution to stay as is. And among the public, 56 percent still oppose any revisions — although that opposition is slowly eroding, according to a recent Pew Research Center survey.

For lawmakers, changing the constitution would require a profound consensus — a two-thirds agreement in both houses. The bar is so high that Abe has talked about first changing Article 96, which stipulates the process for making constitutional changes. Abe would prefer a simple majority in both houses for any change. But even that potential move has proven controversial, and Abe has said little about it on the campaign trail.

The results of Sunday’s election could influence Abe’s approach, some analysts say. The ruling coalition — the LDP and New ­Komeito — need 63 seats combined to gain an upper house majority, considered a near certainty. But if the LDP captures 72 seats on its own, it will have a majority even without its partner, easing the path to any constitutional changes.

Abe has an approval rating near 70 percent, but that will probably dip, pundits say, if he pushes an agenda that is not seen as nearly as essential as his economic policy. In the past seven months, Abe and the LDP have been focused on winning the upper house election, Kochi Nakano, a professor of political science at Sophia University, told reporters during a recent presentation in Tokyo.

“This is one big goal they had,” Nakano said. “Once that’s gone, that can very easily lead to lack of discipline.”