German Chancellor Angela Merkel's party has comfortably won a state election in Saarland, which is seen as an early litmus test ahead of the national vote in September.

Her centre-right Christian Democrats (CDU) took 40.7%, comfortably seeing off a challenge by the Social Democrats (SPD), who secured 29.6%.

Merkel, who has been chancellor since 2005, is hoping to win a fourth term.

The SPD has recently seen a surge in support under its new leader Martin Schulz, with some polls placing the party neck-and-neck with the CDU and even ahead of them for the first time in almost 10 years.

Saarland – a small south-western state with 1m inhabitants – is currently governed by a coalition of the CDU and SPD as junior partner, mirroring the national government.

In Saarland, recent polls had indicated a narrow lead for the CDU, but there had also been predictions that a coalition between the SPD, the far-left Linke and the environmentalist Greens may emerge.

However, Merkel's party's succeeded in growing its vote – up to 40.7% from 35.2% in the last election in 2012. The SPD dropped more than three points in the result while the Greens failed to reach the 5% threshold to enter the state assembly.

The strong result in Saarland for the CDU has brought the celebratory mood from the SPD to an end as realties of the long campaign ahead fall into place.

The state election was the first of three key tests that the political parties will face in the coming months.

Saarland is comparatively small compared to the almost 3m who live in the Danish border state of Schleswig-Holstein, who go to the polls on the 7 May.

But the biggest challenge will be the western state of North-Rhine-Westphalia which has more than 17m inhabitants and on the 14 May will be viewed as a test running of the general election.