A day after Peter Thiel explained why he had given about $10 million to fund multiple lawsuits against Gawker Media, the site’s founder published an open letter to the Silicon Valley billionaire, calling him “thin-skinned” and a “comic book villain” and challenging him to a public debate about the role of journalism in society.

The letter from Nick Denton, the company’s founder and majority owner, was published Thursday on Gawker, hours after the company acknowledged that it had hired an investment banker in recent weeks to help it explore strategic options, including a possible sale.

Mr. Denton harshly criticized what he said was a lengthy, unnecessary vendetta Mr. Thiel waged against not only Gawker Media but also individual journalists.

“This vindictive decade-long campaign is quite out of proportion to the hurt you claim,” Mr. Denton wrote on Thursday. In the letter, Mr. Denton made it clear that he would not back down against a billionaire who showed no sign of stopping his involvement in underwriting the lawsuits, even as the future of Gawker remained in question.