TOKYO—Japan needs to consider building a missile-strike capability against potential foes, including China and North Korea, said one of the two top candidates to become the nation’s next leader, former Foreign Minister Fumio Kishida.

In an interview with The Wall Street Journal, Mr. Kishida struck a hard-line tone on China, calling Taiwan the front line in the struggle by democracies to resist authoritarianism’s advance. He said Tokyo and Washington needed to run joint simulations of how they would respond together in a crisis scenario involving Taiwan, going a step further than current government officials.

The 64-year-old Mr. Kishida is vying to succeed the incumbent prime minister, Yoshihide Suga, who said Sept. 3 that he was dropping out of the race to lead the ruling Liberal Democratic Party after criticism of his handling of Covid-19. The vote for LDP president will be held Sept. 29, after which the winner will be anointed prime minister because the LDP controls a majority in Parliament.

Taro Kono, Japan’s vaccine minister, is expected soon to declare his candidacy to lead Japan.

Photo: Akio Kon/Bloomberg News

The main rival to Mr. Kishida is likely to be Taro Kono, 58, another former foreign minister who now serves as minister in charge of Covid-19 vaccinations. Mr. Kono, who is expected to formally declare his candidacy this week, scored higher than Mr. Kishida among the public in weekend polls by Japanese media organizations, but he has a smaller base of support among senior ruling-party lawmakers.

Mr. Kishida, who served as foreign minister from 2012 to 2017, is the leader of a ruling-party faction that dates to the late 1950s. The faction has a reputation for representing more dovish and liberal views within the party. He represents a district of Hiroshima in Parliament and has campaigned for eventual abolition of nuclear weapons. While foreign minister, he guided then-President Barack Obama around the site of the world’s first atomic bombing.

On security issues, however, he is taking a harder line than the current prime minister, Mr. Suga. Mr. Kishida said Japan couldn’t attack potential enemies first, but he said it needed to be prepared for a scenario in which it was hit first and was expecting further missile attacks.

“Can we protect the lives of the people by watching silently as Japan gets hit?” Mr. Kishida said. “Don’t we need to have the ability to block the other side’s missile attack ability? That’s the problem I’m pointing out.”

He expressed concern about hypersonic missiles that can cruise at low altitude and change direction if needed—such as China’s DF-17. “The other side’s technology is advancing every day,” he said, and updating Japan’s defenses “is a very important political issue.”

Mr. Kishida said China and Russia were at the forefront of the advance of authoritarianism, which he said democracies such as the U.S., Japan, India, Australia and Western European nations needed to combat.

“The front line of the clash between authoritarianism and democracy is Asia, and particularly Taiwan,” he said. He said Japan needed to continually update its preparations for a conflict involving Taiwan. “As a practical matter, Japan cannot respond on its own. We cannot respond except by cooperating with our ally, the U.S. It’s important to conduct simulations for this.”

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Beijing considers Taiwan, a self-governing island, to be part of China, and it says other nations should stay away from the issue—particularly Japan, which invaded China in the 1930s and ruled much of the country until the end of World War II.

“The Japanese side bears historical responsibilities to the Chinese people for its past crimes and should especially be prudent with its words and actions,” foreign ministry spokesman Wang Wenbin said Sept. 3. Mr. Wang called on Japanese political leaders “to refrain from interfering in China’s internal affairs in any form and to refrain from sending wrong signals to ‘Taiwan independence’ forces.”

Write to Peter Landers at peter.landers@wsj.com