It’s not really clear who will form the next government. Everything depends on the Senate. It seems difficult for a minority PD government to form, even supported by the M5S. The reason is technical: in the Senate, abstentions count against the passage of a motion, including a motion of confidence, and so the M5S could not abstain. If it left the chamber (in order to abstain “properly”), the PDL and Lega would also leave the Chamber, depriving the Chamber of quorum. And so, if it wants to support the PD, the M5S must actively vote in favour of it, which it (or rather, Beppe Grillo) has said it will not do.
In all of these debates, it’s important to note how trivial and powerless the Montiani have become. I don’t just mean, trivial and powerless because they only polled less than 10%. 10% is a lot for a “civic list”, albeit one with powerful backers. I mean, powerless because it doesn’t contribute to (m)any winning coalitions in the Senate.
There are some indices of voting power that show this. The Banzhaf index for a party is the fraction of all possible coalitions in a legislative body where that party is pivotal: that is, where it could deprive the coalition of a majority by voting the other way. With eleven ‘parties’ in the Senate, there are 2^11-1 different coalitions, or 2047. The following table shows the percentage of those 2047 in which each party is pivotal, with its seat share next to it.
You can see that the voting power of the Montiani is equal to that of the SVP, despite having three times as many senators. The same is true of the Lega Nord.
Voting power indices like this have many flaws. Because they assume that all coalitions are equiprobable, they don’t take account of ideology. You might think that centrist parties, because of their greater clubbability, have higher empirical voting power than theoretical voting power.
These indices also assume that parties are unitary actors. Maybe, if the PD can peel off enough dissatisfied M5S senators, the Montiani will become relevant. But if you’re reduced to saying that your party becomes important for parliamentary arithmetic only conditional on the meltdown of another party, then you have problems.