The Social Democratic Party (SPD), which is seeking to topple Chancellor Angela Merkel's centre-right coalition in the September ballot, garnered 25 percent in the ARD-DeutschlandTrend survey.
Despite the centre-left party's leading light having presented its policies on Monday, the result marked its lowest level in 21 months, down by a point in popularity from a week previously.
It also means that even combined with its favoured partner the Greens, the two parties together would only score 42 percent compared to 45 percent for Merkel's current coalition.
In the poll, conducted over Tuesday and Wednesday, 1,000 potential voters were asked which party they would vote for if the general election were held Sunday, rather than on September 22nd.
Merkel's conservatives remained at 40 percent but their junior coalition allies, the embattled Free Democrats (FDP), managed to lift their showing by one point to reach the crucial five percent needed to get any seats in parliament.
Plagued by years of internal bickering and evaporating support, the pro-business FDP picked at its party conference Sunday a former economy minister to be its main election candidate as it battles to turn around its fortunes.
Rainer Brüderle, 67, gave a rousing 75-minute speech to the party faithful urging them to "go into battle" to cheers, applause and a five-minute standing ovation.
The survey was conducted by Infratest dimap for ARD public television.