Polls have closed in Germany's regional North Rhine-Westphalia (NRW) elections, the country's most populous state and projections by German media predict a win for Angela Merkel and her Christian Democratic Union.

Current figures presented by German media on Sunday show the conservatives leading with 34 percent with the social democrats at around 30 percent.

The NRW elections are serving as a prelude to September's national vote.

The pressure is on the Social Democrats, led by Martin Schulz, in this election for the state legislature in NRW.

It is Schulz's home territory, though he is not on the ballot, and home to 17.9 million people, nearly a quarter of Germany's population.

The western state, which includes Cologne, Dusseldorf and the Ruhr industrial region, has been led by the Social Democrats for all but five years since 1966.

READ MORE: Germany vote tests public opinion before election

A defeat for centre-left governor Hannelore Kraft would be a major blow for the Social Democrats after poor showings in two previous state elections punctured the party's euphoria over Schulz's nomination to run as a challenger to Merkel in September.

Last weekend, they were beaten by Merkel's party in Schleswig-Holstein in Germany's far north.

Merkel's conservatives in the state, led by challenger Armin Laschet, a liberal-minded deputy leader of the Christian Democrats, had little to lose after a dreadful showing in the state vote five years ago.

They have sought to portray Kraft's state government as slack on security.

They point to burglary statistics, incidents such as the New Year's Eve sexual assaults in Cologne in 2015 and questions over regional officials' handling of sometime resident Anis Amri, the rejected Tunisian asylum-seeker who drove a truck into a Christmas market in Berlin last December, killing 12 people.

They also have attacked what they say is regional authorities' poor handling of education and infrastructure projects.

READ MORE: Merkel's CDU wins elections in Schleswig-Holstein

Kraft's coalition partners, the Greens, are polling poorly and chances of their alliance keeping its majority look poor.

The pro-business Free Democrats, eyeing a return to the national parliament in September after their 2013 ejection, look set for a strong performance.

Meanwhile the nationalist Alternative for Germany, or AfD, hopes to enter its 13th state legislature though its popularity appears to have faded as the migrant influx has receded and the party has been rent by infighting.

After a blaze of publicity earlier this year, Schulz, who chose not to join the government when he returned to Germany after being president of the European Parliament, has struggled to maintain a high profile.

National polls show the Social Democrats trailing Merkel's conservatives by up to 10 points after drawing level earlier this year.