Cornyn's comments came Tuesday afternoon; earlier that day he called on Holder to resign. Sen. John McCain (R-Ariz.) also introduced a resolution Tuesday (which Democrats promptly blocked) calling on Holder to appoint a special counsel to investigate the leaks.
Cornyn said it was "so important to have an investigation of these leaks that was independent, nonpartisan and thorough."
"Unfortunately, Attorney General Holder has demonstrated, at least to me, that he is incapable of delivering that kind of investigation," Cornyn said.
Cornyn cited the background of U.S. District attorney Ronald C. Machen Jr., one of two attorneys Holder appointed to investigate the leaks.
"But instead of an independent prosecutor, Attorney General Holder has decided to appoint two federal prosecutors that are in his chain of command and who are directly under his personal supervision," Cornyn said.
Cornyn said that Machen's background as a former attorney working under then-U.S. attorney Holder — and the fact that he was a donor to President Obama's first presidential campaign — suggested that Machin could not properly conduct a thorough and impartial investigation.
"I don't have any issue any issue on that, that's his right as an American citizen," Cornyn said in reference to Machin's donations to the Obama campaign. "But it does raise legitimate questions about his ability to be independent and conduct the kind of investigation that I'm talking about.
"Oh, by the way, Mr. Machin also got his start as a federal prosecutor when he went to work for U.S. Attorney Eric Holder. That is not an independent investigation. That's the point. And it helps to demonstrate why it is that Attorney General Holder has a conflict of interest himself that requires the appointment of a special counsel, not the appointment of two U.S. attorneys who are directly responsible to him and through whom he can direct the flow of information to members of Congress."
Cornyn's floor speech comes as a distinct partisan divide solidifies over how to handle investigating the leaks. Republicans have strongly called for an independent counsel that would not answer to Holder and would work outside the Department of Justice. Democrats, meanwhile, say that Holder's office could sufficiently handle the investigation. Sen. Dianne Feinstein (D-Calif.) has also said that appointing an outside special counsel could slow down the investigation of the leaks.
"Reasonable people will wonder, where does the attorney general's loyalty lie," Cornyn continued. "To the president of the United States? To try and get him elected? Or his duty to enforce the laws of the United States government?"