"From Europe we have the umpteenth unacceptable interference by unelected officials", League leader Matteo Salvini said on Tuesday.
5-Star's parliamentary party leader Danilo Toninelli told state television Rai: "The only way to get a serious government is to return to the polls".
The two parties have held six days of talks aimed at putting together a coalition government and ending 10 weeks of political stalemate following an inconclusive election on March 4.
"The more they insult us, the more they threaten us, the more they blackmail us, the more desire I have to embark on this challenge", he said.
The Northern League firmly opposes current European Union budget rules. If the agreement is passed, 5-Stars Luigi Di Maio said the details will be distributed over the weekend.
Rhetoric over USA exit from Iran deal rises
Bolton suggested that European allies might agree to new USA approach once they digest Mr. Trump's tough line on Iran , of course, was intended for an audience beyond Tehran.
The League has pledged to introduce a flat tax rate of 15 percent, which would lower tax revenues by some 80 billion euros ($95 billion) per year, according to some estimates, while 5-Star has pledged new welfare payments for the poor at 17 billion euros.
On Tuesday the leaders both hit back hard after European Commissioners said the new government in Rome will have to respect the EU's Stability Pact and hoped Italy's policy on asylum seekers does not change.
Salvini and Di Maio have both agreed to drop their own ambitions to be prime minister and are looking for a candidate from outside their parties to enact their program.
League leader Matteo Salvini told reporters on the way to a meeting with Di Maio on Tuesday evening that there were still areas, including the approach to the European Union, where the parties diverged.
Salvini, saying he was "optimistic but realistic" about a deal, took rhetorical aim via social media at European Union migration commissioner Dimitris Avramopoulos. It might also dismay Italian President Sergio Mattarella, who has repeatedly stressed the importance of maintaining a strong, pro-European stance. However, the League still wants to leave the eurozone as soon as politically feasible. The policy program will probably be published Thursday, 5-Star said.