Clapper, a retired Air Force lieutenant general, received unanimous Senate Intelligence Committee approval last week for his nomination as director of national intelligence. But McCain's "hold" is threatening to keep President Obama's pick from being confirmed by the full Senate until after its August recess, Congressional Quarterly Politics reported.
"Sen. McCain requested a specific report from Mr. Clapper and until that report is provided, Sen. McCain will continue to hold his nomination," McCain spokeswoman Brooke Buchanan said.
She didn't reveal what report the senator is seeking, Congress Daily reported.
Clapper would succeed Dennis Blair, who resigned in May. The acting director, principal Deputy Director David Gompert, is to leave at the end of August.
Clapper's office had no comment on the GOP senator's blocking action, CNN reported.
Senate rules allow members to put "holds" on nominees or bills to signal they might filibuster a vote on the matter until their concerns are addressed, the network noted.