"Voters know Aso lacks leadership," Yoshinobu Yamamoto, a political scientist at Aoyama Gakuin University in Tokyo, said.
"Like previous local polls, the opposition party is likely to win the Tokyo election."
The national election date is still to be announced.
Aso, who took office in September 2008, acknowledged on Friday the Tokyo poll would be "a tough battle".
But he rejected any link between Sunday's vote and the upcoming general election for the powerful lower house of parliament.
"The Tokyo election is a local poll. It is the judgment of various local issues by Tokyo residents," he said at a news conference in the Italian city of L'Aquila where the G8 summit was held.
"As I have repeatedly said, it [poll] is not directly linked to national politics. There is no change in terms of fulfilling my duties as the prime minister."
Opinion polls by Japanese newspapers show that the Democratic Party of Japan is well placed to make major gains or even rise to power in the national election, with opposition leader Yukio Hatoyama likely to replace Aso.
That would mean the end of rule by the Liberal Democratic Party, which has governed Japan for all of the past 50 years except a brief period in 1993.
The Liberal Democrats currently has 303 seats in the 480-seat lower house, and its partner Komeito has 31. The Democratic Party has just 112.
A nationwide survey by the Yomiuri newspaper, Japan's top-selling daily, showed on Friday that 41 per cent of respondents said they would vote for the opposition party in the national election, compared to just 24 per cent for Aso's party.
Nearly 46 percent of respondents said Hatoyama, 62, is fit for the prime minister, compared with 21 per cent favouring Aso.