TEHRAN, Young Journalists Club (YJC) - Polls in Germany's smallest state, focused on the port city of the same name, are tight but indicate the SPD risks losing a stronghold it has ruled for 73 years. No other German state has been ruled by the same party for so long.
Elections to the European Parliament on the same day as Bremen's parliamentary vote could pile further pressure on Merkel's "grand coalition", with both conservatives and SPD likely to suffer heavy losses.
First projections for both are due at 1800 CET (1600 GMT).
If the SPD loses Bremen to the conservative Christian Democrats (CDU), pressure will mount on party leader Andrea Nahles to stand down or break with the federal coalition.
In a sign of growing unrest inside the SPD, German weekly Bild am Sonntag reported that former party leader Martin Schulz wants to replace Nahles as chief.
An SPD spokeswoman said that Nahles and Schulz were in regular discussions, declining to comment on the content of confidential talks.
Bremen has the highest jobless level of any German state. Most recent polls suggest there is scope for three outcomes in Bremen: a grand coalition of the SPD and CDU; a coalition of the CDU, Greens and the business-friendly FDP; or a coalition of SPD with the far-left Die Linke party and the Greens.