An election in the German state of Saarland is being closely watched for signs of how Chancellor Angela Merkel might do in September’s national poll.
It is governed by a coalition of her Christian Democrats and the Social Democrats as junior partner, mirroring the national government.
Mrs Merkel is hoping for a fourth term as chancellor, but recent polls suggest this is less likely than it was.
Polls close at 16:00 GMT in Saarland – a state with one million inhabitants.
The vote in Saarland is also the first major test of the Social Democrats (SPD) under their new leader, Martin Schulz.
The former president of the European Parliament took up the post in January. He is credited with a revival in popular support for his party.
A Deutschlandtrend poll for national TV on 24 March suggested the Christian Democrats (CDU) and the SPD were on level pegging nationally, but that voters favoured Mr Schulz to lead the next government in Berlin.
In Saarland, a recent survey indicated a lead for the CDU, but there are predictions that a coalition between the SPD, the far-left Linke and the environmentalist Greens may emerge.
The populist right-wing Alternative for Germany (AfD), which has played on dissatisfaction with Mrs Merkel’s policy on immigration, could also win seats in the Saarland legislature, as it has already done in several other state assemblies.
The state contest pits Saarland’s prime minister, 54-year-old Annegret Kramp-Karrenbauer, against her deputy in the regional government, the SPD’s Anke Rehlinger, 40.
Mrs Rehlinger, a trained lawyer, has governed alongside Mrs Kramp-Karrenbauer since 2014. She holds the state record in shotput, 16.03m, and was a youth champion in discus.
Mrs Kramp-Karrenbauer has been prime minister of Saarland since 2011.
Saarland, a mainly Catholic state bordering Luxembourg and France, has sought to transform itself into a magnet for hi-tech industry after the decline of coal mining.
The CDU has governed there since 1998.
The national election takes place on 24 September but more state elections, in Schleswig-Holstein and North Rhine-Westphalia, will be held before then.
Germany’s Saarland state in bellwether vote for Merkel