Final answer:
Jefferson objected to the Sedition Act primarily because it infringed on free speech rights and seemed to be enforced in a partisan manner against Republican critics, violating the First Amendment.
Step-by-step explanation:
Thomas Jefferson's primary objection to the Sedition Act of 1798 was that it limited free speech, which he and other Republicans believed violated the First Amendment. The Act also did not protect the vice president from abuse and seemed primarily enforced against editors and authors who were against the Federalist party, signaling a partisan enforcement. Jefferson, along with James Madison, worked through the Virginia and Kentucky legislatures to oppose the Alien and Sedition Acts, leading to the famous Virginia and Kentucky Resolutions. These resolutions suggested that states had the final authority to judge the constitutionality of federal acts and could nullify federal laws deemed unconstitutional within their borders.