The bomb was discovered and defused Thursday night, Italy's ANSA news agency reported.
Investigators are now looking for suspects and possible motives for the threat.
Massimo Bitonci, a League Senator, said that if the bomb had exploded, "It could have been a massacre."
Bitonci said the attack was likely aimed at derailing the Northern League's platform of promoting Italy's northern area and challenging European Union regulations.
"No one will stop us: Obviously someone fears us for our ideas and our battles against the euro and against immigration," he said.