It is my understanding that anybody can challenge constitutionality. The problem lies in the fact that it's an incredibly expensive and long process. As such, only those with sufficient means, or those who have been severely affected by a law, will ever go through the process.

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It's amazing that the Founders missed out on the biggest (and most obvious) flaw in the Constitution... that it's just so easy to ignore. Maybe there should have been a clause that subjects every bill to scrutiny of the courts before becoming law. Obviously, when the Constitution was devised, Congress was more likely to defend the Constitution, anyway, due to the appointment of Senators. Nevertheless, the Constitution was subverted almost instantly: the second President passed laws infringing on the Freedom of Speech (John Adams, Alien and Sedition Act) - the very same people who wrote and voted for the Constitution, wrote and voted for a law that infringed on the most fundamental right in the Constitution.