Bremen is not deemed a political bellwether for other parts of Germany given its specific characteristics as an industrial port city and its small size - it has just 570,000 inhabitants.
By contrast, the states of Bavaria and Hesse, which will hold elections in October, together comprise nearly a quarter of Germany's population of 83.2 million.
Still, the traditional SPD stronghold - a car hub that is turning into a key port for the growing offshore wind industry - is the first federal state to hold an election this year.
An exit poll by ZDF state broadcaster at 1800 local time put the SPD in pole position on 30%, up 5.1 percentage points from the last election in 2019 when it suffered a bitter loss to the conservatives. Despite the defeat four years ago it was still able to retain the mayorship, after forming a coalition with the environmentalist Greens and far-left Left party.
Sunday's result, if confirmed, would be around 10 percentage points better than the SPD is polling on a national level where it has slumped since the 2019 federal election amid a cost of living crisis in the wake of the Ukraine war.
"The number one in Bremen - that's us," said Mayor Andreas Bovenschulte.
The conservatives came in second place in Bremen on 24.5%, down 2.2 percentage points on their 2019 result, according to the ZDF poll.
Meanwhile both parties partnering with the SPD in Scholz's federal coalition, the Greens and liberal Free Democrats (FDP), suffered losses in the Bremen election, according to exit polls, which could heighten tensions in Berlin.
The ZDF poll put the Greens on 12%, down 5.4 percentage points, and the pro-business Free Democrats on 5%, down 0.9 percentage points.
"We need to look at what we did wrong," said Omid Nouripour, Greens co-chief, saying this was also a reflection on the party's performance at national level.
The FDP will be keeping a close eye on the election results as they trickle in on Sunday evening, given they failed last year to pass the 5% threshold to enter parliament in three state votes.
The ZDF poll put the Left party on 11% and the right-wing populist party Buerger in Wut (Citizens in Rage) on 10.5%.
Support for the Buerger in Wut jumped after the far-right Alternative for Germany (AfD), which has surged in nationwide polls to 15-17%, was absent from the ballot in Bremen due to internal divisions.
(Reporting by Sarah Marsh, Andreas Rinke, Alexander Ratz and Victoria Waldersee; editing by Mark Heinrich, Matthias Williams, Ros Russell)
By Sarah Marsh and Andreas Rinke
