Another test is coming next weekend with the elections in Germany's most populous state, North Rhine-Westphalia, which is known as a Social Democrat stronghold.
NRW has been governed by a center-left coalition of the SPD and the Greens since 2010, with the Social Democrat Hannelore Kraft serving as state premier.
Merkel's Christian Democrats (CDU) fetched 33% of the vote in Schleswig-Holstein, up from 30.8% in the last election there in 2012, projected results for broadcaster ARD showed.
Schleswig-Holstein's CDU candidate Daniel Guenther, speaking after projected results were first announced, said that voters had made a clear decision against the incumbent state government, which he referred to as "the coalition of stagnation".
In the two preceding state elections in Saarland and Schleswig-Holstein, Ms Merkel's party was running neck-and-neck or just behind the SPD in the polls during the campaign, only to outperform its biggest rival on polling day.
While the SPD is likely to return to power in their political western heartland next Sunday, a slump among its Green coalition partner could see the smaller party banished to the opposition benches - or even outside the Düsseldorf parliament door.
In September, Germany will go to the polls to decide between Merkel, who is hoping to win a fourth term as chancellor, or the SPD's Martin Schulz, the former EU Parliament president.
Thomas Oppermann, chairman of the Social Democrats in Germany's parliament, called for a swift and thorough repraissal of the SPD following its "heavy defeat" in Schleswig-Holstein.
It dropped below the 5 per cent threshold for legislative representation in the 2013 national election but is expected to exceed that level in September.
Current polls suggest that the SPD may be able to improve on their result from five years ago and maintain power in the state.
A total of 23 percent of respondents said they would not vote or have not decided which party they would support.
The party will nearly certainly lose its place at the head of the Schleswig-Holstein government after it came a disappointing second with just 26 per cent.
Merkel s party has seen a comeback, after losing a string of state elections over the past two years as voters punished the German leader for her liberal refugee policy that allowed more than one million asylum seekers into Germany since 2015.
This year's campaign has seen battles over education, policing and roads, a top concern in a state with a population of 2.8 million thinly spread across nearly 16,000 square kilometres. The survey of 1,814 people put the Greens at 12 percent, the pro-market Free Democrats at 11 percent, the anti-immigration Alternative for Germany at 6 percent, the Left party at 4.5 percent and the Danish minority party at 3 percent.
Another battleground is wind farm construction near residential areas - no small matter in a windy coastal region whose turbines are a key element in Germany's energy transition away from nuclear and fossil fuels.
"He is for an European Union in which responsibilities are standardized and blurred", Lindner said, accusing Schulz's SPD of conspiring with Macron to promote higher government debt.
A win for either party would send a signal to voters and pundits ahead of September's national elections.