Tang, a wealthy businessman and the city's former chief secretary, was believed to have Beijing's backing until a series of personal scandals and gaffes destroyed his standing with the general public.
The South China Morning Post daily cited unnamed sources close to Beijing saying the central government had started to privately lobby for Leung Chun-ying, Tang's chief opponent ahead of Sunday's vote.
"Every candidate has a chance to win or lose," Tang, 59, said when asked to comment on the report.
"My target is on March 25. I will work harder to gain the support from the people in the next four days," he told reporters.
A 1,200-member electoral committee packed with mainly pro-Beijing business and social elites will choose a replacement for outgoing Chief Executive Donald Tsang, whose term expires in June.
Ordinary Hong Kongers do not have the right to elect their leader, but have made their opinion known through approval ratings showing Leung, 57, with a hefty lead over Tang and pro-democracy candidate Albert Ho.
Pro-Beijing Liberal Party chairman James Tien was quoted by the Post as saying he believed Leung had a better than 50 percent chance of winning, after Beijing started to make its intentions clear to committee members.
The mainland authorities, who are in the midst of their own leadership struggles, have not openly backed one candidate or another.
But some analysts took Premier Wen Jiabao's comments last week that the southern financial hub's next chief would have the support of the "vast majority" of the people as a sign that Leung was now Beijing's man.
Hong Kong Baptist University political scientist Michael DeGolyer, who is on the electoral committee and supports Ho, said "for the first time" Beijing officials were paying close attention to what Hong Kong people wanted.
"Beijing officials have said repeatedly that public opinion matters more, that it needs to be listened to," he told AFP.
Hong Kong reverted to Chinese control from British rule in 1997, with a semi-autonomous status that guarantees broad social freedoms under limited democracy.