A key House Republican on Tuesday defended his move to subpoena two Democratic attorneys general over their aggressive efforts to investigate energy companies, and accused them of refusing to comply in order to hide the details of their efforts from Congress.

"Your office, at the behest of various environmental groups, is investigating alleged fraud by Exxon and others, and to that end has issued subpoenas, demanding the production of documents and communications between, among others, Exxon and scientists, both internal and external, who conducted research relevant to the issue of climate change," House Committee on Science, Space, and Technology Chairman Lamar Smith, R-Texas, wrote to the AGs from New York and Massachusetts.

"This research is funded by a variety of public and private sources, including the federal government. Although you have not made your subpoena to Exxon publicly available, it is likely that your demand will include the product of federal funded researchers. Why won't you make your subpoena public? What are you hiding?" he asked.

New York Attorney General Eric Schneiderman and Massachusetts Attorney General Maura Healey have been probing the practices of Exxon and others in an effort to show that they defrauded investors by suppressing information about how burning fossil fuels affected climate change.

Smith issued a subpoena for their work, after which the two AGs said the subpoena was illegal. But Smith rejected that as well in his letter.

"The committee is conducting an investigation to determine whether the actions of your office are having an adverse impact on federally-funded scientific research. If such an adverse impact is discovered, the committee may consider changes to federal law and/or the amount and allocation of federal funding for scientific research," Smith wrote.

"The committee's goal is to maximize the efficient and effective use of federal tax dollars intended to advance the progress of science without regard to non-scientific considerations such as a fear that certain types of scientifically justified research may lead to costly state investigations and adverse political pressure," he added.

Smith also argued that the subpoena issued by the committee fits the three-pronged test set out by the Supreme Court in Wilkinson v. United States: The committee is investigating something with in its purview, the subpoena serves a valid legislative purpose, and it is a pertinent investigation.

He added that the subpoena does not violate the First Amendment rights to free speech and doesn't violate the 10th Amendment, which deals with federalism.

Schneiderman has pledged not to comply with the subpoena, which he views as Smith exceeding Congress' authority by messing with state government affairs.

The group of state attorneys general he leads has been slammed for their investigation into ExxonMobil and affiliated groups that don't believe in climate change. Critics say Schneiderman, Healey and other attorneys general are using the powers of their office to prosecute speech they don't like, namely the belief that climate change isn't real.