Suga, 70, is scheduled to arrive in Washington on May 9 for only his second official overseas trip in six-and-a-half years.

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Two issues will dominate his discussions with US counterparts – obtaining renewed support for Tokyo’s efforts to win the freedom of Japanese nationals abducted by North Korea, and reducing the burden on the people of Okinawa caused by a large US military presence in the prefecture.

These are issues Abe, who is from the Liberal Democratic Party (LDP), previously handled, so Suga’s trip adds weight to the idea that they might one day become his responsibility. In September, Abe was re-elected head of the party, meaning he can serve until 2022, although there is talk he might step down after Tokyo hosts the 2020 Olympic Games.

“Who Mr Suga talks to and whether he is invited to the White House will act as a litmus test for Suga’s future,” an LDP senior official told the Yomiuri Shimbun’s English-language daily.

Within the party there is growing support for a politician who – unusually in Japanese ruling circles – does not come from a political family.

“There is a saying within the Liberal Democratic Party that only Abe can succeed Abe, but if the party must have a new leader then Suga is the right person for the job,” said Ken Kato, owner of a small business and a paid-up supporter of the LDP.

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“When the time comes, I think a lot of people will like the fact he is from a very ordinary and humble background, unlike Abe or finance minister Taro Aso, who are from long-standing political families,” he said. “Suga came to Tokyo and studied at night school to get his law degree and worked his way up in the world of politics through his own hard work and endeavour.

“That’s quite rare in Japan and I think people will have admiration for that,” said Kato, a human rights activist campaigning for the return of Japanese abductees from North Korea. Kato said he once met Suga, who came across as “extremely intelligent”.

“He listened closely to what we had to say, he clearly understood the issue in great detail and was attentive to our suggestions.”

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A staunch supporter of Abe’s government, Kato says Suga could be relied upon to continue the majority of policies that have proved such a hit with conservative voters in Japan, both on domestic and international matters.

Jeff Kingston, director of Asian studies at the Tokyo campus of Temple University, agrees that Suga has served as a “formidable lieutenant” to Abe, but might not walk into the post – even if the present prime minister makes clear Suga is his preferred candidate.

“He has been a great chief cabinet secretary – he can be a pitbull when it comes to serving as the guardian of his boss’ interests and he’s adept at dealing with the media, although I don’t think many people are going to look at him as some sort of champion of press freedom,” he added.

“But there are certainly some other people in the party who might well think they are better suited as prime minister, so any election for party leader might be quite divisive.”

Shigeru Ishiba, a former defence minister, challenged Abe in last year’s party election and might fancy his chances again, while Shinjiro Koizumi, the son of former prime minister Junichiro Koizumi, is popular with the public.

But the 37-year-old Koizumi has expressed his frustration at being viewed as just a young politician, in a society that tends to assess one’s maturity by their age rather than their attributes.

Former internal affairs minister Seiko Noda had indicated her desire to lead the party in the past, while foreign minister Taro Kono may also throw his hat into the ring when the time comes.

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If Suga does want the post, then his close working relationship with Abe for his record-setting administration might prove to be the decisive factor.

“Suga is the brains of ‘Team Abe’ and he is very good at orchestrating and keeping cabinet members in line,” Kingston said. “And the factional politics within the LDP mean he will have numbers in his favour if he can count on Abe.”

Suga’s weakest point could prove to be his lack of charisma – that hard-to-teach attribute that many average politicians in other countries rely on to get into power. Whether he is managing a press conference or announcing the name of the new imperial era, critics say Suga rarely smiles or even shows much in the way of warmth.

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With a shrug, Kingston says a “lack of affability never seemed to hold Abe back”. Said Kato: “In Japanese political culture, the prime minister is more of a coordinator, a planner or strategist for the nation, and public image is less important to the voters.

“Suga has a firm grasp of the sort of management that is required of a Japanese leader. To us, that is what counts .” ■