The far-right League and the 5-Star Movement have been discussing a common policy agenda for a week after a March 4 election ended in a hung parliament.
A new Italian government was put on hold on Monday after the leader of anti-establishment Five Star Movement asked President Sergio Mattarella for a few more days to form a coalition with the far-right League.
In a Facebook video, Di Maio said that the establishment feared change but "the more they attack us the more I am motivated" to reach a deal.
"First must come the things that need doing, and only afterwards (we need worry about) absurd rules written many years ago when the world was totally different", he said.
On Wednesday morning Salvini said that negotiations were nearing conclusion, while Di Maio confirmed the two parties could wrap up the "contract of government" within the day.
They have both vowed to scrap an unpopular pension reform - a move that would punch a 15-billion-euro hole in state coffers.
Di Maio told reporters in parliament "the recipe to lower public debt is by investments and expansionary policies". "We can not go to Brussels with positions that are far apart", he said.
Salvini, saying he was "optimistic but realistic" about a deal, took rhetorical aim via social media at European Union migration commissioner Dimitris Avramopoulos.
"It would be insane to give up at the moment of truth", League leader Matteo Salvini said in a live video stream on Facebook, adding that he would not be intimidated by negative reaction from financial markets or attacks from the media.
Di Maio charged against the Financial Times and unelected Eurocrats for undermining the prospect of an M5S-league government.
5-Star has adopted a softer line on migration and Salvini said on Monday this was another area of discord between the two sides. The policy program will probably be published Thursday, 5-Star said.
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.
5-Star and the League said in a statement that the draft, which was dated May 14, was "an old version that has been considerably modified".