ROME — He’s been called “gray,” “invisible” and “gentle to the point of seeming fragile.” Silvio Berlusconi, the former prime minister, once compared him to “a monk.”

But on Monday, Italy’s quiet, white-haired president, Sergio Mattarella, emerged as the most contentious figure in Italian and European politics. His refusal to confirm a euroskeptic economist as a government minister set off the collapse of a populist coalition hours before it was expected to take control of the European Union’s fourth largest economy.

Mr. Mattarella’s defenders hailed him as the courageous protector of Italy’s democracy, institutions and financial health, while fuming populists sought to make the usually revered figure of the Italian head of state the country’s public enemy No. 1. They called for his impeachment, saying he had overstepped his constitutional bounds with delusions of grandeur, blocked the will of the people and destroyed Italian democracy.

In response, Mr. Mattarella privately plugged along.

On Monday morning, as markets rose and fell with the whiplashing events in Italy, Mr. Mattarella gave a new mandate to form a government to Carlo Cottarelli, a respected economist, former International Monetary Fund official and Italian government appointee, who told reporters that he would form a caretaker government only with the goal of passing a budget and guiding Italy to new elections.