Leung Chun-ying is mobbed by press photographers after handing in nomination papers for the position of Hong Kong chief executive on Feb. 23. (AARON TAM/AFP/GETTY IMAGES)

As a high school student when Britain ruled Hong Kong, Florence Leung Mo-han was taken into an empty classroom by a teacher and asked to take a secret oath of loyalty. Standing alone before the blackboard, she raised her right hand before a small flag of the People’s Republic of China. “I was very moved,” she recalled.

The furtive ceremony marked Leung’s first step into the hidden world of Hong Kong’s underground Communist Party, an organization that, decades later, still functions entirely out of view but that is casting a long shadow over a decision Sunday on who will lead this Special Administrative Region of China for the next five years.

Officially, the party does not exist in Hong Kong, despite the former British colony’s return to Chinese rule in 1997. Across the border in mainland China, the party has more than 80 million members and runs a country of 1.3 billion people. In Hong Kong, though, it is invisible. It is not registered, has no address or telephone number, and has no declared members.

“You can’t see it or touch it,” Leung said. “But it definitely exists.”

So much so that a wealthy land surveyor who she and many others believe is a covert member of the party may become Hong Kong’s next leader. He is Leung Chun-ying, the front-runner to win a vote by 1,193-member local grandees entrusted with filling the top government job. Leung, who is widely known as C.Y. and is no relation to Florence Leung, has repeatedly denied being a member of the party. Nobody has produced solid evidence that he is.

‘Secret’ Communist Party

A thick cloud of suspicion and fearful speculation nonetheless hangs over Sunday’s decision on Hong Kong’s next chief executive. The vote climaxes a tumultuous contest that, though highly undemocratic, has pushed to the fore an issue that touches all of the city’s 7.1 million people: Is Hong Kong to run its own affairs, as promised when Britain and China signed a 1984 agreement for the transfer of sovereignty, or is the party to take command?

“I really have sleepless nights,” said Anson Chan, Hong Kong’s second-most senior official in the dying days of British rule and then in the first years of Chinese control. “People feel they don’t know the real C.Y.,” Chan said, describing the candidate as “a chameleon” who “always puts up a facade.”

She added: “I have never heard him talk about the core values of Hong Kong with any passion — personal liberties, the rule of law, transparency and even economic freedom.”

At a time of deep political uncertainty in Beijing after the recent ouster of Bo Xilai, the son of a revolutionary elder previously seen as a rising star, Hong Kong is swirling with dark rumor and a few hard facts about China’s role as the vote nears.

Allegations that Leung, the son of a policeman, is a closet Communist have circulated for years. British colonial officials, who tried to keep tabs on the party through a police unit known as Special Branch, would whisper in private that Leung, now 57, joined the party as a young man, although they offered no proof.

In recent days — amid signs that Beijing has shifted its support to Leung instead of early favorite Henry Tang — this sotto voce murmuring has become a noisy hubbub of accusation in the news media, on the Internet and in public gatherings. Florence Leung, 72 and now an ex-communist, flew in from Vancouver to publicize a collection of essays about her experiences in the party — and sound the alarm over Leung’s alleged membership. An anti-Leung group popped up on Facebook urging “everyone to spread widely the information that Leung Chun-ying definitely is a Communist Party member.’ ”

Leung, speaking to reporters last weekend, categorically denied being a member of the Party and said he has “never been a member of any political party either ‘underground’ or ‘above ground.’ ” Ta Kung Pao, a Party-controlled newspaper in Hong Kong, denounced a “clear political plot” by “black hands.” And anyway, the paper added, belonging to the Party is nothing to be ashamed of: “Membership of the Party is a glorious title.”

Though officially nonexistent, Hong Kong’s underground party apparatus has been an open secret for decades. Under British rule, it operated under the aegis of the Xinhua News Agency. The nominal press agency served as a base for Chinese intelligence operatives, officials responsible for supervising Chinese business interests and party functionaries who guided a network of pro-China schools, newspapers, trade unions, cultural groups and other bodies. The head of Xinhua in the colony was never a journalist: His real job was to lead the local party, known as the Hong Kong and Macau Work Committee.

Xu Jiatun, Xinhua’s chief from 1983 to 1990, when he fled to the United States, put the number of party members in Hong Kong during his time at more than 6,000, roughly half of them local recruits and the rest mainlanders.

After the handover, Xinhua turned its headquarters overlooking the Happy Valley Racecourse into a hotel and, from a new office block nearby, started focusing on news. Responsibility for the party apparatus passed to the Liaison Office of the Central People’s Government, located in a tightly guarded tower in Hong Kong’s Western District.

Jasper Tsang Yok-sing, a veteran pro-China politician who heads the Hong Kong legislature, said that after taking back Hong Kong, Chinese officials discussed whether to let the party operate openly but “decided it is better to keep underground” because breaking cover would threaten the authority of the Hong Kong government.

He declined to say if he is himself a member, noting that anyone who says yes “probably isn’t” and “if I say no, nobody will believe me.”

Circumstantial evidence

In the run-up to Sunday’s vote, members of the selection committee reported being leaned on by China’s Liaison Office to support Leung, and local news media complained of bullying over their coverage. Chan, the former senior Hong Kong official, said the Liaison Office is “interfering in a blatant way.” The big and worrying question, she added, is “how is Leung going to pay them back.’’

Nobody believes Leung is a Communist intent on ending capitalism — his supporters include a host of millionaires. But critics worry about where his ultimate loyalties lie.

Beijing initially backed Henry Tang, the son of a rich industrialist who is supported by property moguls and some powerful friends in China. But his campaign got mired in a swamp of scandals involving his love life and an illegal underground “palace” at a family villa.

Martin Lee, a prominent lawyer and longtime democracy campaigner, acknowledged that the evidence of Leung’s supposed affiliations is circumstantial: a manner that strikes many as shifty, trips to China when the country was still largely closed, an apparent reluctance to challenge Beijing on anything and an out-of-the-blue appointment early in his career to an important consultative post for the hand­over.

“We all believe C.Y. is a Communist Party cadre,” Lee said. “Of course, he denies it. He has to deny it. It is his job to deny it.”

Florence Leung, the lapsed Communist, noted that for years, she had kept her membership secret. Formally inducted into the party during a visit to Guangzhou in southern China, she signed a membership form and started paying dues, but she got no party card and never knew the identities of any but a handful of other members in Hong Kong, she said.

“Everything was secret,” she said. “Everything.”