Senator Edward Kennedy says an anonymous Republican senator is blocking a bill to ban plastic guns.
Next month, the current ban expires for the weapons that could slip past airport security and metal detectors at other security checkpoints.
Senate rules allow any senator to put a hold on a bill without being named.
Kennedy says no lawmaker should be allowed "to embarrass the entire Senate by blocking action" on the ban.
Lawmakers want to renew the Terrorist Firearm Detection Act of 1988, which outlaws making or owning plastic firearms. Military and intelligence agencies are exempt.