In court, the commission, represented by the office of Ohio’s attorney general, Michael DeWine, asked the court to dismiss the lawsuit on procedural grounds.

But Mr. DeWine, a Republican and a former United States senator, filed his own brief, one that was at odds with that of his office. He acknowledged a duty to defend his state’s laws, and he said he had fulfilled that duty by instructing lawyers on his staff to represent the commission. But he said he himself could not stay silent.

“What really was the final straw for me,” he said in an interview, “is looking at what can happen with social media, with private citizens who all they want to do it express their point of view. It’s a town hall. It’s a public square, really.”

According to Mr. DeWine’s brief, the election commission has found violations of the state’s false-statements law more than 110 times since 2001. Such a finding is typically the end of the matter, but prosecutions are possible. “In a worst-case scenario,” Mr. DeWine said, “there is a six-month jail sentence.”

“In my opinion,” he added, “the law as it is applied and as it is written is blatantly unconstitutional.”

The election commission’s brief relegated the First Amendment to a footnote that cited decisions upholding the Ohio law. In 1991, for instance, the United States Court of Appeals for the Sixth Circuit, in Cincinnati, said that “false speech, even political speech, does not merit constitutional protection if the speaker knows of the falsehood or recklessly disregards the truth.”

In 2007, by contrast, the Washington Supreme Court struck down a similar state law.

“The notion that the government, rather than the people, may be the final arbiter of truth in political debate is fundamentally at odds with the First Amendment,” Justice James M. Johnson wrote.