The Italian experience provides a blueprint for how to defeat Mr. Trump. Only two men in Italy have won an electoral competition against Mr. Berlusconi: Romano Prodi and the current prime minister, Matteo Renzi (albeit only in a 2014 European election). Both of them treated Mr. Berlusconi as an ordinary opponent. They focused on the issues, not on his character. In different ways, both of them are seen as outsiders, not as members of what in Italy is defined as the political caste.

The Democratic Party should learn this lesson. It should not do as the Republicans did after President Obama was elected. Their preconceived opposition to any of his initiatives poisoned the Washington well, fueling the anti-establishment reaction (even if it was a successful electoral strategy for the party). There are plenty of Trump proposals that Democrats can agree with, like new infrastructure investments. Most Democrats, including politicians like Mrs. Clinton and Bernie Sanders and economists like Lawrence Summers and Paul Krugman, have pushed the idea of infrastructure as a way to increase demand and to expand employment among non-college-educated workers. Some details might be different from a Republican plan, but it will add credibility to the Democratic opposition if it tries to find the points in common, not just differences.

And an opposition focused on personality would crown Mr. Trump as the people’s leader of the fight against the Washington caste. It would also weaken the opposition voice on the issues, where it is important to conduct a battle of principles.

Democrats should also offer Mr. Trump help against the Republican establishment, an offer that would reveal whether his populism is empty language or a real position. For example, with Mr. Trump’s encouragement, the Republican platform called for reinstating the Glass-Steagall Act, which would separate investment and commercial banking. The Democrats should declare their support of this separation, a policy that many Republicans oppose. The last thing they should want is for Mr. Trump to use the Republican establishment as a fig leaf for his own failure, dumping on it the responsibility for blocking the popular reforms that he promised during the campaign and probably never intended to pass. That will only enlarge his image as a hero of the people shackled by the elites.

Finally, the Democratic Party should also find a credible candidate among young leaders, one outside the party’s Brahmins. The news that Chelsea Clinton is considering running for office is the worst possible. If the Democratic Party is turning into a monarchy, how can it fight the autocratic tendencies in Mr. Trump?