In general, the Left does not ask the question, "What will happen next?" when formulating social policy. Not thinking through the long-range consequences of their positions is liberalism's tragic flaw.
Take almost any position that distinguishes the Left:
Will higher taxes help the economy?
The major reason the Left advocates tax increases is not that these tax increases will help the American economy. Higher taxes rarely help the economy, and most liberals don't even make that argument. Their argument is about equality, the Left's paramount value. The animating factor for the Left is narrowing the gap between the rich and poor. That is why so few on the Left have had moral problems with Fidel Castro's totalitarian regime -- Cubans may not have liberty, but almost all Cubans are equally poor. Likewise, that explains left-wing support for Venezuela's Hugo Chavez even as he develops into a Castro-like dictator: He advocates economic equality.
Is continued illegal immigration good for America or for Mexico?
Regarding illegal immigration, what most concerns the Left is not the consequences of illegal immigration. It is compassion for the illegal immigrant. Now, I happen share that concern -- were I a poor Mexican seeing no hope for me or my children in my corrupt homeland, I, too, would try to enter America illegally. But it is not enough to have compassion for the illegal immigrant; the responsible citizen needs to consider the consequences of vast numbers of people illegally entering his country. If America is increasingly unable to sustain -- economically, demographically, in terms of crime -- the great number of illegal immigrants, it is incumbent on all responsible people to figure out how to stem the flow of illegal immigrants. It is not even good for Mexico, because it enables that country to avoid needed reforms. Any country that knows its poorest citizens can go to another country from which they will also send back billions of dollars is hardly being pressured into doing anything about its poverty.
Is bilingual education good for immigrant children?
Here, too, compassion trumps effectiveness. The country that has successfully assimilated the greatest percentage of immigrants is Israel, and that country does not have bilingual education. Immigrant children in its public schools are immersed in Hebrew, despite the fact that Hebrew is far more difficult than English is for most of its immigrants (especially those speaking Latin languages). But it is not what works that matters for liberals advocating bilingual education; it is their perception of compassion and multiculturalism.
Does affirmative action help black students?