Taiwan's opposition Nationalist Party has threatened to call a public referendum asking for an independent probe of the election eve shooting last month of President Chen Shui-bian.

A party spokesman Tuesday said the referendum process will begin soon unless the president agrees to set up the impartial investigation. The opposition contends the shooting influenced the election in Mr. Chen's favor. He won by a very narrow margin.

The government has an investigation under way, and a panel of US forensic experts is assisting the probe. But Mr. Chen has said there is no need for a separate investigation.

On Monday, lawyers for the opposition and its defeated presidential candidate, Lien Chan, filed a lawsuit with Taiwan's High Court, seeking to nullify the results of the election and to hold a new vote. It was their second election lawsuit. The first one asks for a recount of the vote.

A three-judge High Court panel heard arguments on the first lawsuit Friday, and ordered both sides to work out details of a recount. Another hearing is set for Wednesday.