Merz's Christian Democratic Union has enjoyed a narrow poll lead over the SPD -- their coalition partners at the national level -- who have ruled the mid-sized state for 35 years.
Polling third is the far-right Alternative for Germany (AfD), which spells a greater threat to the two centrist parties in several state elections in September in the country's ex-communist east.
About half of eligible voters had cast their ballots by midday in the state election. Around 34 percent had applied for postal voting, while another 16 percent voted in person within the first four hours, according to the state election commissioner, DPA reported.
Polling stations close at 6pm on Sunday.Â
The picturesque state of Rhineland-Palatinate, bordering France, Belgium and Luxembourg and with a population of about four million, is known for its steep river valleys, many lined with vineyards and topped by castles.
It is also home to heavy industry, including steelmakers and chemicals giant BASF, and hosts several US military facilities including the sprawling Ramstein Air Base.
The state's SPD premier Alexander Schweitzer, 52, has campaigned in hopes of holding together his current coalition with the Greens and the liberal Free Democrats.
But polls suggest the CDU's top candidate Gordon Schnieder, 50, could put his party back in power there for the first time since 1991.
A CDU victory would be a shot in the arm for Merz, who has faced poor popularity ratings and struggled to meet his campaign pledge of rebooting Europe's biggest economy after years of stagnation.
The CDU's top candidate in Rhineland-Palatinate is the brother of Merz's national transport minister Patrick Schnieder.
"We want to win. And if we win, we'll be happy," Merz said at a campaign event with Gordon Schnieder on Friday.
But he stressed that the CDU win he hoped for over the SPD should not distract the two parties from their government cooperation at the national level.
"The next day I will continue discussions with the SPD party leaders how we can get this country back on track together," he said.
Far-right AfD eyes strong gains
For the SPD, a defeat would spell another blow as the fortunes of Germany's traditional labour party have faded in recent years amid a string of bitter electoral defeats.
Nationally, the SPD hit a historic low point last year, when ex-chancellor Olaf Scholz led them to their worst result in more than a century -- 16.4 percent -- in the February 2025 election.
The insurgent anti-immigration AfD are in third place in Rhineland-Palatinate, polling at close to 20 percent, more than double the support the party won in the last election in 2021.
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Nationally, the AfD is now neck-and-neck with the CDU, with both polling at around 25 percent.
In September, voters in eastern Germany will take part in several state elections, with the AfD expected to perform particularly well and potentially claim outright majorities.
Any alliance with the AfD remains taboo in the country, with all other parties refusing to cooperate with the far right.
The CDU faced a bitter defeat earlier this month in the larger state of Baden-Wuerttemberg, where missteps squandered a large lead in the polls to allow the Greens to claim victory.
READ ALSO: German state election deals blow to Merz as Greens claim victory
In Rhineland-Palatinate, the Greens are expected to come in fourth, according to the polls, which have roughly tracked their 2021 result of nine percent.
In the absence of a clear majority for any party, a coalition between the CDU and SPD appears to be the most likely scenario for the region.
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