The Social Democrats climbed 4.5 percentage points to 33.7 percent, according to the Sifo poll for March. The party has climbed 9.1 percentage points since former union head Stefan Löfven was appointed as leader.
"I have never seen such a sharp change over two consecutive months. But one has to assume that this pace will not hold," said Toivo Sjörén at Sifo to the Svenska Dagbladet daily.
The Moderates are the main loser in the poll, falling by 4.2 percentage points to 28.7 percent.
The Social Democrats' success in the poll is explained by a direct transition of voters from the Moderates and is not at the expense of the Green Party or the Left Party.
Political scientist Jonas Hinnfors' explained the dramatic change in Stefan Löfven moving the Social Democrats towards the centre.
"He is positive towards nuclear power and as soon as he was appointed the Social Democrats reached an agreement with the government on the euro pact," Hinnfors said.
The changes in support for the Social Democrats and the Moderates are the only statistically significant changes in the March poll.
The embattled Christian Democrats fell 0.3 points to 3.5 percent and would thus fall below the threshold for parliamentary seats. The Centre Party dipped 0.2 points to 4.9 percent and the the Liberal Party (Folkpartiet) fell 0.6 points to 5.9 percent.
Both the Green Party and the Left Party declined marginally in the poll, by 0.2 and 0.6 points to 10.3 and 6.1 percent respectively.
The Sweden Democrats increased by 0.5 points to 5.2 percent, but would lose their position of kingmaker according to the poll.
The Sifo poll interviewed 1,918 voters between March 5th-15th, asking the question "Which party would you vote for if a parliamentary election were held today".