People shout slogans and hold up red placards reading "violation of the constitution" and blue ones that read "scrap security bills” at a 2015 rally in Tokyo over plans to expand the military’s role. (Tory Yamanaka/AFP/Getty Images)

Japanese Prime Minister Shinzo Abe's push to revise the  American-written, postwar constitution has hit its first stumbling block, with his own party unable to agree on how to change the clause that has kept the country pacifist for the past seven decades.

Even as lawmakers within the ruling Liberal Democratic Party wrangle with how to update the war-renouncing Article 9, the prime minister is pressing ahead with efforts to strengthen Japan's military by buying American weapons, as encouraged by President Trump.

Abe's defense minister has requested a budget totaling $46 billion, 2.5 percent higher than last year. A big part of the increase would be to buy American-made long-range cruise missiles capable of striking enemy bases — a controversial move while Japan is still bound by the constitution to act only in defense.

Now, following his victory in snap elections in October, Abe is pushing ahead with a long-cherished desire to revise the constitution, which was drawn up under the U.S. occupation forces after the country's defeat in World War II.

"Abe's efforts at constitutional revision would be a watershed moment in an ongoing, decades-long process in which conservatives have moved Japan in the direction of what they see as a more 'normal' country," said Jennifer Lind, a Japan expert at Dartmouth College.

The efforts are also controversial in the region. China probably will protest the changes as Japan "remilitarizing," so Abe should take care to frame constitutional revision as a way to ensure the balance of power in the region, Lind said.

"If Abe and other conservatives discuss constitutional revision amidst rhetoric about 'making Japan great again,' that would not only alarm China, it would alarm a lot of people in the region and the world — including those who are currently predisposed to trust Japan," she said.

In his five years as prime minister, Abe has led an effort to "reinterpret" the constitution to allow Japanese troops to fight overseas to aid the United States, its ally, and has permitted some military exports.

Now, he has directed a special committee to come up with a draft revision of the constitution.

Its adoption would require the support of two-thirds of both houses of the Diet, or parliament — both of which the LDP and its junior coalition partner Komeito control — and approval by a majority of the public in a referendum. 

The most controversial part is the two-paragraph Article 9. The first paragraph says, "the Japanese people forever renounce war," while the second says Japan will not maintain "land, sea, and air forces, as well as other war potential."

As a result, Japan does not technically have a military, instead operating "self-defense forces" that are allowed to act only if the country comes under attack.

Abe wants to add a third paragraph to this article, to recognize and legitimize the existence of the self-defense forces.

Even within the LDP, there are sharp divisions over that proposal, said Hajime Funada, acting chairman of the LDP's constitution revision panel.

"We can't come up with a draft that everybody agrees with," Funada said in an interview in his office. Almost two-thirds of LDP members support the prime minister's idea to add a third paragraph, but the remainder say it would contradict the ban on "war potential."

The LDP is trying to forge a consensus among its members before taking its proposal to Komeito, a party with Buddhist roots that has had to compromise its pacifist principles in government.

Many in Komeito question why the constitution needs to be revised.

"We don't really feel the need to revise  Article 9 now, and we're wondering if there's any urgency," said Kazuo Kitagawa, who leads Komeito's panel on constitutional issues. "We don't consider the self-defense forces to be unconstitutional, and I think most Japanese don't, either."

With the LDP struggling to reach a consensus, let alone persuade its junior coalition partner, the effort has fallen behind Abe's ambitious schedule to revise the constitution by 2020.

The political calendar contains plenty of potential complications. Abe will need to be reelected as head of his party next year. Then in 2019, ailing 83-year-old Emperor Akihito will be permitted to abdicate, the first time in more than 200 years that the emperor has stepped down, and upper house elections are due to be held by July.

In 2020, Tokyo will host the Summer Olympics, a much-anticipated event for the Abe government.

"Getting the timing right will be very difficult," said Sota Kimura, a constitutional law professor at Tokyo Metropolitan University.

The government will not want to disrupt the emperor's abdication process, but will also not want to run the risk of losing its two-thirds majority in the upper house, he said.

"So I'm sure there are people who feel the need to hurry and propose the draft revision to the Diet before then," Kimura said.

Funada, the LDP representative, said a momentous change such as revising the constitution should not be rushed to fit a political timetable.

"We need to come up with a good compromise," he said, adding this would also help it win broader public support.

Opinion polls show the public is almost equally divided on Abe's plan. A survey by the conservative Yomiuri newspaper found that just under half supported the prime minister's desire to mention the self-defense forces in the constitution, but 40 percent did not. The proportion of those in favor of the change has steadily ticked up as North Korea has launched more missiles toward Japan.

When the LDP-Komeito coalition used its supermajority to "reinterpret" the constitution in 2015, there were huge protests on the streets of Tokyo.

"If we try to push this through the Diet with the support of the ruling parties only, we might get a result in the national referendum that is opposite to the one we want," Funada said, raising the prospect of the revision being done in stages.

In the meantime, Abe's government is pushing the limits of the constitution and trying to strengthen the self-defense forces. 

In the coming week, Abe is expected to approve a Defense Ministry request for funds to acquire three kinds of air-to-surface cruise missiles, two of them American made, which would dramatically expand the range of Japan's Air Self-Defense Force.

During his visit to Tokyo last month, Trump urged Japan to buy "massive" amounts of American military equipment to shoot North Korean missiles "out of the sky."

The government insists those weapons are intended strictly for defensive purposes, and it would take years to modify Japan's fighter jets to fire the missiles. But acquiring the potential capability to launch offensive strikes could stir domestic political controversy, said Tobias Harris of Teneo Intelligence, a consulting firm. 

Under its reinterpretation of the constitution, Japan could argue it is acting in self-defense by launching a preemptive strike on a North Korean missile base, Harris wrote in a research note.

Yuki Oda contributed to this report.