Although the state bordering France is home to just one million people, its vote half a year before national elections is seen as a test of the Social Democrats' rising fortunes under new leader Martin Schulz.
The SPD, having long played second fiddle to Merkel in a national right-left grand coalition, has been re-energised since the folksy and plain-spoken Schulz became its chairman in January.
The former president of the European Parliament has lifted party support by 10 percent with promises to help the socially disadvantaged and end Merkel's almost 12-year reign in September elections.
The SPD is currently the CDU's unhappy junior partner in Saarland and in the national government -- and in both cases hopes to grab power by teaming up with other leftist parties.
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Even if the CDU comes out ahead in Saarland, the SPD could potentially seek a coalition with the far-left Linke and possibly the ecologist Greens parties -- a so-called "red-red-green" alliance.
The same trio is now running the city-state of Berlin, although policy hurdles remain at the national level, given that the Linke, for example, rejects German membership of NATO.
The incumbent is popular CDU premier Annegret Kramp-Karrenbauer, 54, often dubbed simply "AKK", who is considered pragmatic and unpretentious, dressing up as a cleaning lady at carnival festivities.
Her SPD challenger is deputy premier Anke Rehlinger, 40, who happens to hold the state record in shot put (16.03 metres).
Polls opened at 0600 GMT, and about one third of eligible voters had cast their ballots by 1200 GMT, slightly more than in 2012 elections, suggesting turnout for the day could be higher than the 62 percent five years ago.
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