By Yoko Wakatsuki, James Griffiths and Joshua Berlinger, CNN

(CNN) -- A decision by Japanese Prime Minister Shinzo Abe to call a snap election appears to have paid off.

Abe's ruling coalition has won a clear majority with more than two-thirds of Parliament's 465 seats, with the Liberal Democratic Party holding a majority even without its coalition partner, the Komeito party, he told reporters Monday.

"We were able to earn the powerful support of the Japanese people, well surpassing our goal," Abe said at a press conference after Sunday's vote.

The win puts him on course to be post-war Japan's longest-ever serving prime minister, and Abe is expected to use his new mandate to push for overhauls to the country's defense strategy and pacifist stance.

"My immediate task is to deal with North Korea," Abe told reporters late Sunday. "It will take tough diplomacy. With the mandate given by the people, I would like to exercise my command in diplomacy."

US President Donald Trump called Abe to congratulate him Sunday night, the Japanese Prime Minister's office told CNN. During the call, the two leaders spoke of being united on the need to increase pressure on North Korea, an official said.

Earlier this year, in the face of an increasingly hostile North Korea, Abe set a deadline of 2020 to revise Japan's constitution, which contains language that bans the country from maintaining armed forces. It is a controversial proposal that strikes at the heart of the country's post-war identity.

Abe called the election in order to secure a two-thirds "super majority" in Parliament, which will help him achieve his goal of amending the constitution.

An exit poll by public broadcaster NHK forecast a decisive win for Abe's coalition, saying his conservative Liberal Democratic Party (LDP) would take 253 to 300 of Parliament's 465 seats. The Komeito party is predicted to take 27 to 36 seats.

Official results are still to be released after Typhoon Lan prevented some people from voting in parts of the country, particularly those on outlying islands. However, their votes aren't expected to change the outcome.

"I must face with this victory with humility," Abe told NHK after the exit poll results. "The voters gave us, the ruling party, a majority. This is the voice of the Japanese people, telling us to push our policies forward and come out with results."

Opposition in disarray

Abe's election call was highly tactical, coming at a time when the opposition was in disarray, and Abe's personal popularity rating was beginning to pick up after months in the doldrums.

The conservative Party of Hope launched by Tokyo governor Yuriko Koike splintered the traditional opposition but failed to make much of a dent in Abe's victory.

Koike did not herself run for a seat in Parliament as some had predicted she might, while her party failed to put forward a viable candidate.

Speaking to reporters in Paris, where she was Sunday on a scheduled official trip, Koike said voters had delivered "a very harsh verdict for my party."

"I had expected Hope to serve as a rallying point to counter (Abe's) continued predominance in power. Regrettably, the party has become a target of criticism instead," she said, appearing at times to be on the verge of tears.

"It is a very tough result. We will analyze the reason for the defeat."

Koichi Nakano, a professor of Japanese politics at Tokyo's Sophia University, said Koike "basically overplayed her hand."

"She initially thought she had a real chance of becoming prime minister by launching her party and taking over the Democratic Party," he said, referring to the traditional opposition force in Japanese politics that split into Koike's party and the left-wing Constitutional Democratic party (CDP).

Nakano said a "purge" of more liberal members from the Party of Hope's rolls left it with few credible candidates and hurt Koike's standing with some former Democrat voters.

"The problem is that Koike is not really an opposition politician," he added, pointing to her history in Abe's LDP.

While some, particularly on the left, had bemoaned the weakening of the opposition, Nakano said the emergence of the CDP as more popular than Koike's party has left "what looks like the nucleus of a more competent opposition maybe shaping around (CDP President Yukio) Edano."

State broadcaster NHK put CDP's seat count on 54 -- compared to just 49 for Koike's party -- making it the largest opposition force in the Diet.

North Korea threat

Japan has faced increased hostility from North Korea in recent months, as the rogue state locks horns with the Trump administration over Pyongyang's missile and nuclear programs.

Pyongyang flew two projectiles over Japan last month, including a ballistic missile, in act of defiance against Washington and the international community.

But Japan's ability to respond is limited. Its constitution does not allow Japan's military -- called the Self-Defense Forces -- to develop significant offensive capabilities. Article 9 of the constitution says that "land, sea and air forces, as well as other war potential will never be maintained."

Neighbors that suffered under Japanese occupation during World War II view Abe's ambitions with concern and skepticism. It's also highly controversial in Japan itself, to the point where lawmakers came to blows debating the issue in 2015.

But altering Japan's constitution is a heavy lift. Even with strong parliamentary support, such a measure must be approved in a referendum.

The Trump factor

One of the first items on Abe's agenda will be US President Donald Trump's first trip to Asia while in office early next month.

Trump's talk of turning inward has had US allies in the region worried that they can no longer rely on the US for defense as they once did. Some Japanese hawks have pointed to this as yet another impetus to revise the constitution.

Abe has invested significant time and effort into his relationship with his new US counterpart, becoming the first foreign leader to meet with Trump when he was President-elect and during a summit in February at Trump's Mar-a-Lago estate, where the two played golf together.

Today, the two are bound by their shared hard line on Pyongyang -- both have been vociferous in their opposition to North Korea's missile and nuclear tests.