Laschet voted in the normally secret polls for his and Merkel’s Christian Democratic Union (CDU) in his hometown of Aachen.
The election rules dictate that the ballot “must be folded in a way so that the choices cannot be seen” so as not to influence other voters.
The faux pas before live cameras set off a Twitter storm, with some calling it an “own goal” for Laschet, although electoral commission chief Georg Thiel later said the ballot was still valid.
“A nationally known politician voted, as expected, for his own party,” Thiel tweeted. “This would not influence voters.”
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Laschet is trailing his Social Democratic (SPD) competitor Olaf Scholz, the centrist finance minister and vice chancellor, by about three points after a campaign marred by repeated missteps.
After capturing a narrow lead in the early summer, he was caught laughing on camera during deadly flooding in the west of the country in July as President Frank-Walter Steinmeier gave a solemn speech in memory of the victims.
The widely criticised incident sent support for the CDU plunging, allowing the SPD to pull into the lead.