Those soundings have also shown a plunge in support for the oft-bickering 5-Star Movement, whose sometime left-leaning platform has kept the League from pulling the government further to the right.
With parliament in summer recess, Salvini has summoned all League lawmakers back to Rome on Monday and is pushing for a vote on the motion as early as next week, while opposition parties would prefer to wait until August 19-20.
The prime minister added that he had assured Salvini that he would ensure the transparency of the ongoing crisis.
Prime Minister Giuseppe Conte said on Thursday that Salvini should explain to Italians why he wants to bring down the coalition government.
Salvini said his party would run for elections alone, and if it fell short of the 40% majority required to govern it would "choose a travel companion".
Conte, who is not affiliated with either party, has accused Salvini of wanting an early vote to allow the League to profit from now favorable voter sentiment, with opinion polls showing the party as being likely to comfortably win any election in the coming months.
Its leader Luigi Di Maio cancelled all his previously scheduled engagements on Thursday and accused Salvini of playing "political games" by seeking to obtain more cabinet ministers even though Salvini denies that is his goal.
"The only thing I am interested in is that we fix a date for the election".
Salvini said in a statement on Thursday he had told Prime Minister Giuseppe Conte, who belongs to neither coalition party, that the alliance with 5-Star had collapsed after barely a year in power and "we should quickly give the choice back to the voters".
In the European election in May this year, however, the League took the most votes in Italy with 34 percent against about 17 percent for M5S.
Tensions within the coalition of Salvini's right-wing League party and the populist M5S have been high recently over an infrastructure project involving a high-speed rail link with France.
The head of state, President Sergio Mattarella, is the only person with the power to dissolve parliament, but he could first try to form a new government from within the existing legislature before resorting to elections.
Opinion polls suggest the League could win 38% of the vote in a fresh election, putting it within reach of controlling both houses of parliament without a coalition partner.
"The goal of the next government is to set a 15 percent tax rate for many Italians".
"We're going to cut taxes for workers and businessmen", Salvini told a rally in the south-eastern Apulia region which was streamed live on Facebook on Friday evening.
"The political chaos in Italy is complete and it's linked to the failure of this government majority", said Letta - the former head of the PD - in an impromptu interview at Lamezia Terme airport in southern Italy.
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