By Madeline Chambers and Paul Carrel
BERLIN (Reuters) - Germany's Social Democrats bled support in Bremen on Sunday, failing to win most votes in the state for the first time since 1946 in a blow that could hasten the end of its national coalition with Chancellor Angela Merkel's conservatives.
The centre-left Social Democratic Party (SPD) won only 24.5% of the Bremen vote, according to an exit poll on ARD television, down from 32.8% in 2015. Bremen is Germany's smallest state but second place represents a significant setback in the SPD's northern bastion.
Merkel's Christian Democrats (CDU) led in Bremen with 25.5%, the poll showed, leaving the Greens as potential kingmaker by virtue of an 18% share of the vote.
The SPD's electoral decline will intensify pressure on party leader Andrea Nahles to stand down and possibly break with the federal coalition.
"The loss of its pivotal role in Bremen is another manifestation of the SPD’s seemingly endless decline," said Carsten Nickel, managing director at consultancy Teneo.
"In Berlin, the calls for quitting Merkel’s grand coalition will only get louder."
Both the SPD and Merkel's conservative bloc suffered losses in European elections on Sunday, bleeding support to the Greens in a further test for their loveless coalition government.
An exit poll in the European Union's biggest member state showed the Greens on 22%, the CDU and their conservative allies on 28%, down eight percentage points from 2014, and the SPD slumping nearly 12 points to 15.5%.
Many among the SPD's rank and file are fed up with serving as Merkel's allies. It has proved a thankless role that the party has undertaken in 10 of the past 14 years, leaving the chancellor to steal the limelight, especially on the international stage.
The party reluctantly re-entered a Merkel-led coalition last year after slumping to its weakest support since 1933 in the 2017 federal election.
The SPD is due to review the coalition by the end of the year and pressure from members could grow to instead reinvigorate the party's leftist roots in opposition.
Such a move could force a snap federal election or an attempt to forge a different coalition.
(Additional reporting by Markus Wacket, Andreas Rinke and Christoph Steitz; Editing by David Holmes, Keith Weir and David Goodman)