[marktwain]

The US Constitution requires that any loss of liberty (the freedom to do whatever doesn't violate the rights of others) be imposed by due process of law--which means that it must be imposed as a sentence rendered by a court pursuant to conviction for a crime. Punishments imposed directly by a legislature--especially including ones "tacked on" after a court has already imposed a sentence--are Constitutionally prohibited. The reason is because such laws are Bills of Attainder, and are explicitly forbidden by the Constitution (a prohibition that applies to both Congress and the States.)

If this is true (I always thought it was), how did the Supreme Court reconcile ruling that the Lautenberg amendment, which removes your right to possess a firearm or ammunition if you were convicted of a misdemeanor crime of domestic violence long before the amendment was passed, as Constitutional?

[sourcery]

how did the Supreme Court reconcile fill in the blank as Constitutional?

The same way they ruled that a right to privacy trumps the right to life, or that the power to regulate commerce means that Congress can criminalize the growing of crops on one's own land for one's own personal consumption. In other words, by being legislators, and not judges.

The Supreme Court isn't always right. And over the last 7 decades, they've been making wrong decisions that are clearly wrong way beyond any reasonable doubt ever more frequently--and often ignoring stare decisis in the very same decision.