In 1835, the U.S. Supreme Court in Barron v. Baltimore unanimously ruled that the Bill of Rights did not apply to the states.
During post-Civil War Reconstruction, the Fourteenth Amendment was adopted in 1868 to rectify this condition and to specifically apply the whole of the Constitution to all U.S. states.
Later Supreme Court justices found a way around these limitations without overturning the Slaughterhouse precedent: they created a concept called Selective Incorporation.
"The test usually articulated for determining fundamentality under the Due Process Clause is that the putative right must be 'implicit in the concept of ordered liberty', or 'deeply rooted in this Nation's history and tradition.'"
That process has extended more than a century, with the free speech clause of the First Amendment first incorporated in 1925 in Gitlow v New York.
However, any action that abridges a right deemed fundamental, when also violating equal protection, is still held to the more exacting standard of strict scrutiny, instead of the less demanding rational basis test.
Following the 1937 Supreme Court decision in West Coast Hotel Co. v. Parrish, though, the right to contract became considerably less important in the context of substantive due process and restrictions on it were evaluated under the rational basis standard.