* Scottish nationalists gather with independence in sight
* Narrowing in opinion polls makes September vote hard to call
* Campaigners targeting Scots who don't usually vote
ABERDEEN, Scotland, April 10 (Reuters) - Despite a drumbeat of warnings from businessmen, financiers and British government officials that an independent Scotland would be doomed to failure, Scottish nationalists go into their spring conference this weekend with banners flying and polls showing support for their dream is gathering pace.
Under the conference slogan "Forward," the ruling Scottish National Party (SNP) will hammer home its message that oil-rich Scotland can be a prosperous nation that decides its own future if it votes on Sept. 18 to end the 307-year tie with England.
A year ago, the chance that Scotland's 5.3 million people would vote to leave the United Kingdom looked slim, with support for a break stagnant at just 30 percent.
But opinion polls this year have narrowed the gap, buoying the SNP before the two-day gathering in the oil capital Aberdeen, its last big meeting before the referendum. The "No" to independence side has warned against complacency.
"Everyone now realises that at the very least it will be close," Blair Jenkins, a former TV executive who is heading the "Yes Scotland" campaign for independence, told Reuters.
"In the course of this long campaign the concept of independence has become normalised. Whatever the result in September, we now have a generation hooked on discussing policies and what kind of society we can be."
Political experts said the SNP, which dates back 80 years, has focused on policy issues - with the none of the national identity debates seen in other breakaway aspirants such as Quebec in Canada or Catalonia in Spain.
Key policies under discussion include how independence would affect the economy, making Scotland nuclear-free, European Union membership, and the currency. The main British political parties reject a Scottish government proposal to share the pound.
Two polls released on Thursday confirmed the gap between the Yes and No sides has narrowed to about 13 percentage points from 22 points a year ago.
A Survation poll found voters planning to back independence had dropped two points to 37 percent in the past month while support for a No vote was down to 47 percent from 48 percent.
A separate poll by Panelbase carried out for the Yes Scotland campaign put support for a Yes vote at 40 percent compared to 45 percent for staying in the United Kingdom, with 15 percent of voters still undecided.
"The Yes side has made progress this winter and the race is narrowing but to how narrow we still don't know," said John Curtice of the University of Strathclyde.
"Activists will be hoping that (SNP leader Alex) Salmond gives the speech of his life. It is certainly not an opportunity he can afford to waste."
Salmond will address the party conference on Saturday.
Pro-union "Better Together" campaign chief Alistair Darling, a Scot and a former British finance minister, said the latest polls showed Scots want to remain part of Britain and would rather opt for more powers to be given by London to Scotland's devolved parliament than go for independence.
The UK parties oppose Scottish independence, arguing that the United Kingdom is better and stronger together.
"It's increasingly clear that people in Scotland want to have the best of both worlds - a strong Scottish Parliament, with the guarantee of more powers - backed up by the strength, security and stability of being part of the UK," Darling said.
The narrowing of polls has made the final five months vital for both sides with campaigning in full swing in town halls, streets and schools across Scotland and on social media.
Michael Keating, professor of Scottish politics at Aberdeen University, said a big factor for the vote could be turnout on the day, which was expected to surpass traditional political elections.
He said the Yes side was very active on grassroots campaigning, targeting Scots who would not normally vote, such as those in lower socio-economic groups.
"That could matter. In the small towns of Scotland this could make a big difference," Keating told Reuters.
The British finance ministry said on Thursday Scotland would have the second-highest budget deficit among advanced economies at the likely date of independence - the latest in a stream of reports from the ministry arguing against the split.
Last month University of Glasgow researchers forecast that Scotland would have a budget deficit of 5.5 percent of GDP in 2016, when independence would be likely to take effect.
This is around 2 percentage points higher than estimates from the Scottish government in September, which the Glasgow researchers said significantly underestimated the pace at which tax receipts from North Sea oil and gas would decline.
An independent Scotland would be likely to inherit most of Britain's oil and gas reserves, which lie off its coast, but this previously large source of tax revenue is now in steady decline as reserves become costlier to extract. (Reporting by Belinda Goldsmith, Editing by Angus MacSwan)