Italy is potentially the next domino to fall, partly because of the disillusionment of young voters. They have been swept up by many of the same forces that led peers in Spain and Greece to vote for upstart parties, the British to vote to leave the European Union, and Americans to elect Donald J. Trump. In France, President François Hollande announced on Thursday that he would not seek re-election — another establishment figure succumbing to the political moment.

Mr. Renzi’s supporters have taken to calling his opponents in the internet-born, populist Five Star Movement “Trumpisti.” They accuse their opponents’ numerous blogs and websites of flooding the Facebook accounts of young people with anti-Renzi, pro-Russian fake news. The referendum has essentially become a referendum on Mr. Renzi, who gave extra motivation to his political enemies by vowing to resign if voters reject the proposed political changes…

If Mr. Renzi does step aside, it could open the way for opponents who have threatened to carry the Continent’s fourth-largest economy out of the euro currency zone.

Such a prospect could destabilize the shaky Italian economy; have serious implications for Italy’s troubled banking system, which has been in crisis for a decade; and have a contagion effect around the eurozone.