TOKYO — Fumio Kishida has spent years trying to emerge from the shadow of Shinzo Abe, the former prime minister of Japan who was gunned down at a campaign rally on July 8.

Ever since both were elected in 1993 to the Diet, as Japan’s Parliament is known, Mr. Abe had been the more prominent politician. A charismatic presence, he outshone Mr. Kishida, a party stalwart who can be so stiff that a schoolgirl recently asked him about the last time he truly laughed. (His answer: whenever his beloved baseball team, the Hiroshima Toyo Carp, wins.)

After Mr. Kishida finally — on the second try — ascended to the prime minister’s office, Mr. Abe continued to niggle him from the sidelines. He floated controversial ideas, like a proposal that Japan host American nuclear weapons, and warned that financial markets might see Mr. Kishida’s economic policies as “socialist” and react badly to them.

Now, after the assassination, Mr. Kishida, 64, is trying to honor Mr. Abe while proving that he can set himself apart from the legacy of Japan’s longest-serving prime minister.

“A couple of years ago, Kishida was almost considered as one who had no chance to become prime minister,” said Mikitaka Masuyama, a professor of political science at the National Graduate Institute for Policy Studies in Tokyo. Now, he said, “we have to figure out whether Kishida really has the ability and leadership qualities to run the government and control” his Liberal Democratic Party.

The looming question for Mr. Kishida is how he will spend his political capital, bolstered by a victory in elections to the Upper House of Parliament a week ago. The prime minister had already indicated that he would move to enact Mr. Abe’s most cherished goals, including a revision of the pacifist clause in the Constitution that renounces war, as well as an increase in defense spending.

Last week, Mr. Kishida was quick to say he would take up the “difficult issues” that Mr. Abe had “poured his passion into” but “couldn’t accomplish.” He promised to “drastically enhance Japan’s defense capabilities within five years.”

As much as Mr. Abe’s death, geopolitical circumstances will dictate Mr. Kishida’s choices. The war in Ukraine and rising military threats from China and North Korea have prompted Mr. Kishida, who had previously cast himself as a liberal-leaning, dovish member of the Liberal Democrats, to take on a more hawkish mantle.

Given the regional pressures, “raising defense spending is not optional anymore for Tokyo,” said Titli Basu, an associate fellow at the Institute for Defense Studies and Analyses in New Delhi.

Most Japanese recognize those threats: in polls, a majority backs increasing the defense budget. And while the public once vociferously opposed revising the pacifist Constitution, surveys in the spring indicated that a majority would now consider it.

Mr. Kishida is “saying things that in the past, whoever said it would have had political division,” said Rahm Emanuel, the United States ambassador to Japan. “There is consensus-building that is partly to his credit, and partly to events.”

In the nine months since the party chose Mr. Kishida as prime minister, he has steadily extended the unstinting diplomacy that was a hallmark of Mr. Abe’s reign.

He has also quietly differentiated himself from his predecessor.

When Russia invaded Ukraine in February, Mr. Kishida strongly condemned Russia’s actions without hesitation and swiftly enacted sanctions. Eight years earlier, Mr. Abe, keen to foster a relationship with President Vladimir V. Putin, had dragged his feet on imposing sanctions after Russia annexed Crimea.

Like Mr. Abe, Mr. Kishida came to politics as the son and grandson of members of Parliament.

As young lawmakers who entered the Lower House in the same year, Mr. Kishida and Mr. Abe sometimes worked as a pair. Shinobu Konno, a political commentator, recently recalled on ANN News, a Japanese television network, that the two traveled to Taiwan on a diplomatic mission in 1997, with Mr. Abe as the head of the group and Mr. Kishida as his deputy.

“Mr. Kishida was a strong drinker but a boring talker,” said Mr. Konno. “And Mr. Abe was a good talker but not a strong drinker, so they divided their responsibilities. Mr. Kishida was in charge of drinking and would compete with the stronger Taiwanese drinkers, while Mr. Abe was in charge of talking and getting everyone excited.”

During Mr. Abe’s brief first stint as prime minister from 2006 to 2007, Mr. Kishida served as state minister in charge of Okinawa and Northern Territories affairs. After Mr. Abe returned to power in 2012, he appointed his old friend as foreign minister, a post that Mr. Kishida would hold longer than anyone else in Japan’s post-World War II history.

But when Mr. Abe resigned in 2020, he threw his weight behind another man, Yoshihide Suga, to succeed him. Mr. Suga beat Mr. Kishida by a party vote of nearly four to one.

Mr. Kishida started out by trying to distinguish himself from Mr. Abe, offering a “new capitalism” as a departure from Mr. Abe’s well-known economic platform, dubbed “Abenomics.” Mr. Kishida said he wanted to narrow income inequality and proposed raising some taxes.

He has since ratcheted back that rhetoric, and he has seemed to embrace Mr. Abe’s calls for doubling defense spending and amending the Constitution.

Still, analysts see glimmers of Mr. Kishida trying to be his own man.

Giving the keynote speech last month at a security forum hosted by Singapore, he noted that Germany had announced it would raise its defense budget to 2 percent of its annual economic output — a goal that Mr. Abe had sought for Japan. But Mr. Kishida did not cite a numerical target, instead pledging a “substantial increase.” What’s more, he said Japan would “proceed within the scope of our Constitution.”

Yuki Tatsumi, director of the Japan Program at the Stimson Center in Washington, said she saw Mr. Kishida as “pushing back on some of the stuff that Abe was pushing on him in the court of public opinion.”

As recently as Thursday, Mr. Kishida, referring to defense spending, said that “we must be realistic and concrete in our discussions but at the same time, not be numbers-oriented.”

Economic reality may undercut the possibility of setting drastic targets. With inflation rising, the yen depreciating, coronavirus infections increasing and, in the longer term, the population aging and the birthrate falling, Mr. Kishida may find he doesn’t have the money to pay for all government priorities.

Japan’s traditional pace of change may be on Mr. Kishida’s side. Consensus-building is valued, and incremental progress — rather than radical transformation — is the norm.

“It has been a slow evolution over time where the increasing chipping away by North Korea and China at Japanese security has increased awareness in the public and the politicians that more needs to be done,” said Jeffrey Hornung, a senior political scientist at the RAND Corporation specializing in Japanese security and foreign policy. “As long as Kishida continues to go slow and steady, I do think he’ll be OK.”

Makiko Inoue contributed reporting.