The pro-euro lobby, dogged by criticism that only Sweden's business and political elite wants the euro, got a boost from Wanja Lundby-Wedin, head of the LO union which has two million members out of the nation's seven million voters.
Lundby-Wedin co-wrote an article published on Monday with two fellow union leaders in which she ditched a long-standing policy of neutrality and threw her weight behind the euro.
The unions had held their tongues over the euro as the government would not agree to create special funds to subsidise jobs in an economic downturn once the exchange rate and interest rate cushions are gone.
The government later agreed to adopt other fiscal policy measures to protect Swedish workers, oppening up the way for the union leaders to speak out in favour.
Opponents have led in every opinion poll since April because many Swedes are concerned that adopting the euro would erode the country's generous welfare policies and that Sweden would import the euro zone's higher unemployment.
"Higher trade means more jobs, higher incomes and therefore also higher tax inflows. We get better conditions to safeguard and develop welfare"
Head of the LO union
But Lundby-Wedin argued that a "yes" vote on 14 September would increase trade. "Higher trade means more jobs, higher incomes and therefore also higher tax inflows. We get better conditions to safeguard and develop welfare," she wrote in the Dagens Nyheter daily in the article co-signed by the heads of the Saco and TCO trade unions.
Supporters got a second boost on Monday with publication of a Gallup poll showing the "no" vote's lead had halved to eight percentage points.
The poll of 1019 Swedes, conducted 25-28 August, showed euro support increasing to 36% from 32% in the previous Gallup poll, while euro sceptics lost ground to 44% from 48% . Undecideds were unchanged at 20%.
The Swedish crown rose to four-week highs, trading at 9.1662 to the euro at 1005 GMT against 9.1815 at Friday's close. The crown gains on news that is positive for the "yes" camp because the market believes the currency's eventual pegging rate to the euro should be around 8.8 crowns.
But political scientists said the sudden gains for the "yes" side needed to be confirmed by more surveys to be credible. Financial markets are already pricing in a "no" on 14 September.
"One should be careful with single polls like this one. If the 'yes' side gains a few points in other surveys, then we can talk of a trend," said Rutger Lindahl, a professor of political science at Gothenburg University.
But he said the Gallup poll gains could be a reflection of the stepped-up "yes" campaign, which last week enlisted the help of the respected chief executive of Swedish telecoms equipment maker Ericsson, Carl-Henric Svanberg.