Why is Marbury v. Madison a landmark
case?

The Marbury v. Madison case introduced the model of diffuse control of constitutionality of laws, the judicial review, into modern constitutionalism, basing the Supreme Court of the United States on the principle of supremacy of the Constitution.

As a way to break with the English tradition of Parliament's sovereignty, from 1803, in the famous case Marbury v. Madison started to admit, on American soil, the judicial review or the constitutional jurisdiction, which was, therefore, the intention of the Judiciary to assume the right to control the constitutionality of the laws.

The judicial review is a legacy of genuinely jurisprudential conception and had as its starting point the judgment of the mentioned case, not least because the American Constitution does not provide for the existence of constitutionality control in its text.

to oppose fdr’s new deal

explanation:

the american liberty league. the american liberty league was a nonpartisan organization formed in 1934 in opposition to the new deal. the organization's stated goal was “to defend the constitution and defend the rights and liberties guaranteed by that constitution.”