Alex Salmond has been advised to address his television audience directly and temper his optimism about the case for Scottish independence with a sober, statesmanlike tone when he takes on Alistair Darling in their first televised head-to-head this week.

The first minister's aides believe he must focus on persuading more than half a million undecided Scottish voters to back independence when he faces Darling, the leader of the pro-UK Better Together campaign, for the potentially defining contest on STV on Tuesday.

Salmond's advisers want him to balance his optimism with restraint and to use the debate as a platform to target the 15% or so of Scottish voters who pollsters estimate remain unpersuaded about the case for leaving the UK.

"His opportunity isn't to knock Alistair Darling around the ring; it's to communicate with the audience," said one of Salmond's inner circle.

With opinion polls showing the yes vote is still lagging behind the pro-UK vote, Salmond is facing the greatest test as the campaign enters its final, decisive phase before the referendum is held on 18 September.

A poll by Survation of 1,000 Scottish voters late last week for the Mail on Sunday suggested the race has tightened, to reach 46% for no and 40% yes, with 14% undecided. But Survation, which historically has recorded the highest yes vote of the pollsters, found that the yes vote had fallen by one point since its last poll, while the no vote was static. That implies the yes campaign has not benefited so far from any bounce in patriotism or Scottish self-confidence from the highly successful Commonweath Games in Glasgow.

At the closing press conference for the Games on Sunday, Salmond accused the Liberal Democrat leader, Nick Clegg, of hypocrisy in a row over an interview with the Observer by Nicola Sturgeon, Scotland's deputy first minister.

Sturgeon was cautious about claiming that Scotland's record-breaking results in the Games would convert voters to independence, but told the Observer they could have a clear impact on the country's self-confidence, adding to momentum for a yes vote: "I think it will inevitably leave a feelgood factor".

Clegg, on a visit to Edinburgh, described her remarks as "gauche", leadingwhich Salmond to retort that for the deputy prime minister to describe anyone as gauche "must be the quote of the year".

The first minister suggested Clegg instead use his influence to ask David Cameron to finally accept the yes campaign's challenge to a debate on independence. The prime minister has repeatedly rejected those calls. Salmond and Darling are in talks with the BBC, Sky and Channel 4 about further debates.

With six weeks to go before the poll, the Scottish and UK governments are to send rival booklets on the case for and against independence to all 2.5m households in Scotland this week, just as the Scottish National party prepares to launch its own £1.3m independence campaign.

Holyrood's 12-page booklet, which has been written only because the UK government sent an earlier anti-independence pamphlet to all homes in June, tells voters that Scotland has all the ingredients for a successful, independent country.

It adds that the vote in September is "about all of us having the confidence that our rich country can also be a better society".

The SNP has meanwhile been launching direct attacks on Darling in the runup to the televised debate, accusing him of opening the floodgates to the privatisation of NHS services by the Tory-led coalition government by overseeing the liberalisation of NHS policies in England when he was chancellor.

The yes campaign claims that NHS privatisation in England will threaten health services in Scotland if there is a no vote, even though the NHS is totally devolved to the Scottish parliament and the two services are separate.

Anti-independence campaigners have leapt on warnings from Scotland's largest engineering employer, the defence contractor Babcock, that thousands of its 6,000 employees in Scotland could lose their jobs at Faslane and Coulport nuclear submarine bases and at Rosyth dockyard if there is a yes vote.

A staff briefing said there was a risk that Coulport, the UK's nuclear warhead base, would close‚ while it was unclear whether the firm would stay at Faslane if it became Scotland's non-nuclear naval base.

Salmond, one of the UK's most experienced and skilled debaters, is seen as the clear favourite to win against the more understated Darling, but is under the greatest pressure to use the debate as a platform to close the gap in the polls.

Darling's advisers are trying to carefully manage expectations about his chances on Tuesday, in an attempt to increase pressure on Salmond and lessen the impact of a clear victory for the SNP leader. "Alistair only has one gear," said one member of his team.

There have been discussions within Salmond's camp about how sober he should be and how aggressive he should be with Darling.

The first minister has had fresh training from a longstanding adviser, executive coach Claire Howell, who works for big companies and English Premier League managers, on presenting an upbeat, optimistic vision to voters and avoiding nationalistic, partisan rhetoric.

Some advisers have disclosed their belief that Salmond should also admit there will be challenges and uncertainties from a yes vote, to show a more candid face to doubtful voters, but argue that the rewards of independence are a far greater prize.

Blair McDougall, the chief executive of Better Together, indicated that Darling would be focusing on the unanswered questions in Salmond's independence blueprint on the future of the pound, pensions and public spending.

Referring to the Survation poll, McDougall said: "The pressure is on the first minister to finally give people honest answers to these fundamental questions when he debates with Alistair Darling.

"This poll will do nothing to silence the ticking noise inside Alex Salmond's head as he sees the time running out for him to win over undecided voters."

• This article was amended on Monday 4 August 2014 to change the headline, removing the word "sober".