MEXICO CITY — There is a debate underway within the Democratic Party over what kind of candidate can beat Donald Trump in 2020. A centrist candidate will attract moderate Republican voters, but perhaps demobilize young, minority, college-educated Democrats. A more exciting, perhaps more radical candidate, will mobilize Democrats but scare away moderate Republicans.

From the perspective of a citizen of the country that has probably suffered most under the Trump administration’s policies, the debate signals a historic shift. The rooting of a more Social Democratic identity for the Democratic Party may mean more, in the long run, than defeating Donald Trump in 2020. This is the most interesting and seductive aspect of this presidential campaign. The last two Democratic debates revealed how the party’s center of gravity has shifted to the left: the more liberal members seem increasingly Social Democratic and the more moderate ones, increasingly liberal.

The Social Democratic movement first emerged in Germany in the late 1800s under Otto von Bismarck, the country’s first chancellor. It proliferated and flourished in Western Europe as an antidote to the violence of the Russian Revolution, the emergence of totalitarian Communism and the destruction wrought by two world wars.

In Europe, and later in Latin America, governments placed a greater emphasis on the role of the state in regulating market economies, protecting the weakest sectors of society, seeking to reduce poverty and inequality as much as possible under a capitalist model, defending the environment and strengthening labor unions, workers’ parties and progressive institutions.