Mr. Berlusconi was able to govern Italy for as long as he did mostly thanks to the incompetence of his opposition. It was so rabidly obsessed with his personality that any substantive political debate disappeared; it focused only on personal attacks, the effect of which was to increase Mr. Berlusconi’s popularity. His secret was an ability to set off a Pavlovian reaction among his leftist opponents, which engendered instantaneous sympathy in most moderate voters. Mr. Trump is no different.

We saw this dynamic during the presidential campaign. Hillary Clinton was so focused on explaining how bad Mr. Trump was that she too often didn’t promote her own ideas, to make the positive case for voting for her. The news media was so intent on ridiculing Mr. Trump’s behavior that it ended up providing him with free advertising.

Unfortunately, the dynamic has not ended with the election. Shortly after Mr. Trump gave his acceptance speech, protests sprang up all over America. What are these people protesting against? Whether we like it or not, Mr. Trump won legitimately. Denying that only feeds the perception that there are “legitimate” candidates and “illegitimate” ones, and a small elite decides which is which. If that’s true, elections are just a beauty contest among candidates blessed by the Guardian Council of clerics, just like in Iran.

These protests are also counterproductive. There will be plenty of reasons to complain during the Trump presidency, when really awful decisions are made. Why complain now, when no decision has been made? It delegitimizes the future protests and exposes the bias of the opposition.

Even the petition calling for members of the Electoral College to violate their mandate and not vote for Mr. Trump could play into the president-elect’s hands. This idea is misguided. What ground would we then have to stand on when Mr. Trump tricks the system to obtain what he wants?

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?

Note: This a reprint of an article by Luigi Zingales appearing in The New Yorker..