TAIPEI, May 30 (AP) -- Taiwan's opposition leader said Tuesday that corruption scandals swirling around President Chen Shui-bian are only the tip of an iceberg, and promised a campaign to oust the president if he was involved in any wrongdoing.

Nationalist Chairman Ma Ying-jeou, the front-runner in the island's 2008 presidential elections, spoke several days after police arrested Chen's son-in-law Chao Chien-min on suspicion he used insider information to profit on the purchases of shares in partly state-owned property company Taiwan Development Corp.

That scandal followed widely bruited opposition charges that Chen's wife, Wu Shu-chen, received free vouchers from the management of an upscale Taipei department store, and may have played a role in its takeover by a businessman whose bid was less than that of at least one rival suitor.

"We're only seeing the tip of the iceberg," Ma told reporters. "We will be seeing more. I understand that the people are doing the muckraking, they have a lot of cases in stock."

Ma said if it can be shown that Wu and Chen were involved in illegal actions, he wouldn't hesitate to launch a move to recall the president _ a step that requires a two-thirds majority in the legislature and a referendum passed by at least half of Taiwan's registered voters.

"It is a very difficult task," he said. "But if the president and his wife are involved, I think people's reactions could be very strong."

Ma said a key tipping point in any possible recall drive would be a sense among members of Chen's ruling Democratic Progressive Party that his continuation in office was doing the party harm in the run-up to Taiwan's presidential elections in 2008.

"This hasn't happened yet," he said. "And without the support of DPP members there is no chance (for a recall initiative)."

Under Taiwan's constitution, Chen would be succeeded by DPP Vice president Annette Lu if a recall move succeeded.

Lu is one of three or four DPP stalwarts frequently mentioned as a possible presidential candidate.

Chen, who was elected to his second four-year term in 2004, is constitutionally barred from running again.

Since the Taiwan Development scandal broke earlier this month, his approval ratings have sunk to about 20 percent, the lowest level during his six years in office.