TAIPEI, July 31 (Reuters) - With barely 10 months to go before his second four-year term ends, Taiwan President Chen Shui-bian hopes a planned referendum on U.N. membership will help him avoid becoming a lame duck by shifting the blame to China.

China has claimed sovereignty over Taiwan since their split at the end of the Chinese civil war in 1949. The self-ruled democratic island insists it is a sovereign nation, but only about two dozen countries recognise it compared to 170 for China.

Taiwan has failed in repeated attempts to join the United Nations but Chen hopes the referendum will remind voters that the island's diplomatic isolation is due to bullying by Beijing, and that the opposition favours closer ties with China.

The referendum, which still needs the approval of election authorities, is part of Chen's gambit to boost the chances of his Democratic Progressive Party (DPP) to win the 2008 presidential election and secure his post-presidency role as defender of Taiwan's de facto independence.

But the move is testing China's patience and has irritated the island's main arms supplier, the United States.

"Some argue that provoking a PRC 'overreaction' is precisely Chen's aim," Alan D. Romberg wrote in China Leadership Monitor, referring to Beijing's official name, the People's Republic of China.

"It would score a political three-in-one 'hat trick' by reinforcing his authority in what might otherwise be a lame-duck period, giving a boost to the DPP in the upcoming LY and presidential elections, and rally international support to his cause," Romberg wrote. LY refers to Taiwan's parliament, the Legislative Yuan.

NOT CONVINCED

Chen has argued that the referendum has "nothing to do" with a change in the island's official name -- a promise he made to the United States when he first took office in 2000.

But the United States and China are far from convinced.

China has denounced Chen as "scum". The referendum timing is sensitive for the Chinese Communist Party, which holds a key five-yearly congress in the autumn and whose leaders cannot afford to be seen as weak in the face of perceived provocation.

"China views the referendum as crossing the red line," said Lin Chong-Pin, an former deputy defence minister, who was quoting Chinese academics.

"China has told the United States: 'If you don't do something, we'll do something'," Lin said, adding that Washington was worried that China would respond militarily.

Frank Hsieh, the DPP's presidential standard-bearer, returned home on Monday from a trip to the United States where he had little success in changing Washington's mind on the issue.

Washington opposes any moves that would unilaterally change the political status quo between Taiwan and China.

After surviving opposition attempts to oust him and corruption scandals plaguing his relatives, Chen applied twice in the past month for Taiwan to join the United Nations.

The United Nations, which ousted Taipei in favour of Beijing in 1971 under a "one China" policy, rejected the bid. Taiwan stands virtually no chance of rejoining as Beijing is a veto-wielding permanent member of the Security Council.

The DPP is also toying with the idea of revising its charter in August to adopt a "normal country resolution" and planning a massive rally in September to drum up support for the U.N. bid.

DPP Chairman You Si-kun has defended Chen, saying it would be "tantamount to suicide" if nothing were done to assert the island's sovereignty.