TOKYO — For five years, Japan has defied the skeptics, mounting an eye-popping economic comeback from more than a decade of doldrums under reforms ushered in by Prime Minister Junichiro Koizumi.

But what comes next is a big question mark as the nation prepares to pick a new prime minister. Whoever next pilots the world's second-largest economy faces the tough task of keeping it clipping along and not sliding back into the morass.

The ruling Liberal Democratic Party chooses a new party president tomorrow, and the winner is virtually guaranteed to be elected prime minister when the parliament convenes on Sept. 26.

The new premier will face a host of economic challenges: taxation problems, a growing gap between rich and poor, massive government debt, a shrinking population, frayed relations with Asian trading partners and a lingering oversensitivity to U.S. downturns.

With Japan finally emerging as an engine for global growth, any misfires on these or other challenges could have far-reaching fallout.

"The real issue is whether you get somebody who tries to consolidate those improvements and actually turns it into a more fundamentally dynamic economy," said Richard Jerram, an economist at Macquarie Securities Ltd. in Tokyo.

"And the chances of that at the moment seems fairly low," he said.

Koizumi has been one of Japan's most popular and longest-serving prime ministers for good reason.

Under his watch, unemployment has dropped, the stock market has rocketed, the government has been streamlined and bad debt has been mopped up at the nation's banks. The public corporations running the nation's highways have been privatized, and he passed a plan to do the same to the post office, which also offers broad financial services. Corporate profits have climbed, and Japan's economy has now expanded for six straight quarters.

Hoping to fill Koizumi's shoes in the Liberal Democratic Party presidential vote are Chief Cabinet Secretary Shinzo Abe, Finance Minister Sadakazu Tanigaki and Foreign Minister Taro Aso. Aso is the long-shot, and Tanigaki the man with economic credentials, but Abe is far-and-away the front-runner.

Economic reform is a catch phrase of all the campaigns, with Abe kicking off his race with the pledge to "keep the flame of reform alive." But in reality, the topic has taken a back seat to foreign affairs, education and talk of reforming the constitution.

Abe says he will boost Japan's economy by developing human resources, encouraging innovation and transparency and luring fresh foreign investment. He wants to further cut government spending, but is vague about how to raise badly needed taxes.

Tanigaki calls for doubling the sales tax to 10 percent. Aso, a comic book enthusiast, meanwhile plans to champion Japan's animation industry as an avenue to growth.

But economists say there are big problems for the successor — like how to afford the spiraling healthcare and pension costs of Japan's aging population.

"The most important thing is healthcare and pensions, and it's on nobody's agenda," said Jesper Koll, chief economist for Merrill Lynch in Tokyo.

Another foreboding hurdle is debt. Japan's outstanding government debts total more than 170 percent of gross domestic product, which is the largest such ratio among developed countries, according to the Organization for Economic Cooperation and Development. That will have to be reined in while finding other revenue sources.

To sustain long-term growth, Japan must also balance its economy so it is not as sensitive to foreign export markets.

Unlike past recoveries, Japan's current rebound seems to be getting a boost from consumer spending at home.

But the most recent data show that Japanese retail sales slipped for the first time in three months in July, while data released last Tuesday indicated consumer sentiment fell in August, prompting the government to conclude that optimism was "weak" for a third straight month.

The country's central bank is also experimenting with raising interest rates for the first time in six years, after leaving it at virtually zero percent for most of that time. Some business leaders and politicians worry hiking rates too fast could snuff out the recovery.

Further complicating his task will be a growing gap between rich and poor.

While economists write off the disparities as an inevitable outgrowth of a slimmed-down government and a more efficient economy, the gap is increasingly a source of social and political friction in a country that long prided itself on economic parity.

Abe has virtually no experience in economics or finance. Yet he is likely to pick up where Koizumi left off because he has a similar market-oriented philosophy, Morgan Stanley chief economist Robert Feldman said.

"Policy continuity is highly likely in an Abe government," he said.