* Scotland to hold secession vote on Sept. 18
* Up to a million voters still undecided - polls
* First TV debate to take place on Tuesday
* Nationalist leader a polished media star
* Two-hour debate begins at 8 p.m. (1900 GMT)
By Alistair Smout
EDINBURGH, Aug 5 (Reuters) - Supporters of Scottish independence are hoping leader Alex Salmond’s performance in a U.S.-style television debate on Tuesday can boost their campaign, which is lagging in opinion polls.
With just over six weeks to go before Scots vote on whether to break Scotland’s 307-year union with England, the “No” campaign holds a steady and substantial lead over the “Yes” camp.
However, some polls suggest that as many as a quarter of Scotland’s 4 million voters remain undecided.
The television debate, the first of the campaign, pits Salmond, fast-talking leader of the pro-independence Scottish National Party, against Alistair Darling, the head of the “Better Together” campaign.
Darling was also finance minister in the last British Labour government, led by fellow Scot and “No” campaigner Gordon Brown.
Pundits say Salmond, 59, the most senior politician in Scotland’s devolved government and the driving force behind the “Yes” camp, is favourite to win the two-hour debate, despite the gap in the polls, because of his powerful rhetorical skills.
Darling, 60, is viewed as a safe and steady, albeit somewhat uninspiring, representative for the “No” campaign, although he and the Labour party command considerable authority in Scotland.
The latest opinion poll, by Survation, showed that 46 percent of voters would back the “No” campaign and 40 percent will vote “Yes”, with 14 percent saying they hadn’t decided. Some polls show up to a quarter undecided.
Glasgow’s successful staging of the Commonwealth Games, which closed on Sunday, had been seen by some as a chance to attract more “Yes” voters but appears to have had little impact.
The “Yes” campaign says Scotland, which has its own parliament but lacks substantial tax-raising powers, would be freer, better governed and richer if it went it alone.
The “No” campaign argues Scotland would be unable to keep the British pound, that tens of thousands of jobs in the defence and financial sectors would be at risk, and that an independent Scotland might find it hard to rejoin the European Union.
On the morning of the TV debate, Britain’s three main national political parties all said they would seek further powers for Scotland in the event of a “No” vote, in the areas of fiscal responsibility and social security.
Salmond has long demanded a TV debate with British Prime Minister David Cameron.
But the British government has said that Cameron, like other people in England, does not have the right to vote in the referendum and so it would be inappropriate for him to debate with Salmond.
Cameron has warned that a vote for independence would undermine Britain’s global clout and imperil its financial and political stability.
However, he has taken a back seat during the campaign, conceding, according to people close to him, that his centre-right politics - his Conservatives hold just one of 59 Scottish seats in the British parliament - and his privileged background mean he isn’t the best person to win over Scots.
The debate will be shown live on TV in Scotland, but not in England. (Additional reporting by William James; Editing by Andrew Osborn and Susan Fenton)