He maintained that when he returned to Taiwan and became then President Chiang Ching-kuo's English secretary, his U.S. residency was not against any government regulation of the time.

"President Chiang Ching-kuo never asked about the (green card) issue," said Ma, who served as the late leader's aide in 1988.

"Neither did I mention it specifically to Mr. Chiang nor intend to hide the fact from him, because holding a green card was not against any government regulation at the time."

The KMT hopeful reiterated that his green card was invalidated long ago.

He challenged his rival candidate, Frank Hsieh, from the ruling Democratic Progressive Party to make public any evidence, if any, to prove that he had done anything illegal.

But Hsieh said had Ma told Chiang about the green card, the president "absolutely" would not have employed him.

Asked if he would expose more information about the Ma case in two days as he had promised, Hsieh said he is screening the information he has been given, and verifying some of it.

"There may be some delay in making it public, but it is a very serious matter in that whether a person who would become the president should hold a green card," said Hsieh.

Questioning Ma's loyalty to Taiwan, Hsieh claimed a green card holder could catch a plane and leave the island anytime while others were being stranded without a visa to go anywhere during times of turmoil.

"He said his green card was invalid. How do you know it is really invalid?" said Hsieh, adding he still has to verify the KMT rival's claims.Hsieh dropped a bombshell by revealing that Ma and his wife were green card holders, while their older U.S-born daughter is a citizen of both Taiwan and the United States.

Ma has maintained that his green card is no longer valid because his application for a non-immigrant visa to go to America has automatically canceled his U.S. resident status.

But Hsieh has demanded Ma make public all relevant documents, including the non-immigrant visa, to show that he has really lost his U.S. permanent residency.

Results of a latest survey conducted by the TVBS satellite channel showed that 45 percent of the respondents did not think the green card issue would affect Ma's campaign, while 27 percent said it would have slight impacts.

A total of 52 percent though Ma was honest, while 32 percent thought Hsieh was honest and the same percentage of respondents thought he was dishonest.

Ma's support dropped three percentage points to 53 percent, while Hsieh's support remained at 26 percent, the survey showed.

DPP legislative leader Ker Chien-ming likened the Ma case to the incident that dogged James Soong's independent presidential campaign in 2000.

Soong was then alleged to have been involved in a financial crime, but he failed to provide a clear explanation to convince the nation that he was not involved, Ker claimed.

The DPP legislator said Ma is now also failing to clear all doubts hanging over his honesty.

"This incident is posing a challenge to Ma's integrity and his management capability in general," Ker said. "How can we trust him with the presidency?"

DPP Legislator Chiang Chao-yi questioned whether Ma had failed to file his tax returns with U.S. authorities after returning to Taiwan.

U.S. law requires all citizens and permanent residents declare all their overseas income, he said.

Ma's spokesman Lo Chih-chiang maintained that the candidate's green card is "invalid as a matter of fact."

"The Hsieh campaign should stop playing up the loyalty issue. Holding a U.S. green card is different from U.S citizenship," Lo said.

KMT Legislator Kuo Shu-chun said Ma is unquestionably loyal to Taiwan.

But she also demanded Hsieh explain some of increase in his wealth, as shown in his annual declaration.