John McLaughlin refrained from giving any details on Wednesday, but likened the threat reports to those that preceded the 11 September 2001 attacks.
In an interview with National Public Radio, the acting director confirmed Washington had no specifics on the timing and targets but a "conviction" as big as the one it had before 9/11.
"And the reason I say that it is serious is that I think the information I've seen is very, very solid. We have very little doubt about the information we have in terms of its sourcing and its authenticity," he said.
Homeland Security Secretary Tom Ridge warned last week that "credible reports" indicate al-Qaida plans to carry out a large scale attack in the US to disrupt the coming elections.
But Ridge also said precise knowledge of the time, place and method of attack was lacking, but security is being stepped up at the sites of the Democratic and Republican nominating conventions this summer.
Confidence in prediction?
Former CIA director George Tenet stepped down last weekend after seven years at the head of the agency, which has been criticised for its inaccurate intelligence on Iraq's alleged WMD.
A US Senate report released last week also said most of the major key judgments in the intelligence community's October 2002 National Intelligence Estimate (NIE) on Iraq's suspected arms were either overstated or incorrect.
However, in a CNN interview, McLaughlin defended the agency's work on illicit weapons.
"I tell the American people that they should have confidence in the American intelligence community," he said.