Mr. Smith has called the state-level investigations a violation of scientists’ right to free speech. Mr. Smith said he had called Wednesday’s hearing to “affirm the legitimacy” of his inquiry.

The attorneys general and the organizations have refused to comply with the subpoenas from Mr. Smith, as they have refused to comply with earlier demands for documents from him, claiming that the federal subpoenas are unconstitutional. The attorneys general have cited case law going back to the proceedings of the House Un-American Activities Committee and have cited principles of states’ rights — an argument usually made by conservatives. Mr. Schneiderman and his supporters say the First Amendment argument is a smoke screen to avoid a legitimate investigation.

The subpoenaed organizations, including the Union of Concerned Scientists and 350.org, have also refused to comply. This week, a group of more than a dozen First Amendment scholars and litigators sent a letter to Mr. Smith, arguing that his subpoenas, not those from the state officials, were a threat to the organizations’ First Amendment rights, and that they exceeded the legal authority of the committee.

Critics of Mr. Smith, who has questioned the overwhelming scientific consensus underlying climate change, note that he has received more than $675,000 from the fossil fuel industry since 1998, including more than $24,000 from Exxon Mobil. Supporters of Mr. Smith and Exxon Mobil launched a counterattack over the weekend, noting that Mr. Schneiderman has received substantial campaign contributions from people and organizations with an interest in environmental matters.

The witnesses called for the hearing included Ronald D. Rotunda, a professor at Chapman University’s law school, and Elizabeth Price Foley of Florida International University College of Law. Both are affiliated with conservative causes and organizations: Mr. Rotunda has ties to the Heartland Institute, which disputes climate science, and Ms. Foley recently wrote an article for The Wall Street Journal in support of Mr. Smith’s subpoenas.