The Campaign of 1800 is earlier, and nastier than 1824.
The 1800 election was a re-match of the 1796 election. The campaign
was bitter and characterized by slander and personal attacks on both
sides. Federalists spread rumors that the Democratic-Republicans were
radicals who would ruin the country (based on the
Democratic-Republican support for the French Revolution). In 1798,
George Washington had complained "that you could as soon scrub the
blackamoor white, as to change the principles of a professed Democrat;
and that he will leave nothing unattempted to overturn the Government
of this Country". Meanwhile, the Democratic-Republicans accused
Federalists of destroying Democratic-Republican values, not to mention
political support from immigrants, with the Alien and Sedition Acts,
some of which were later declared unconstitutional after their
expiration by the Supreme Court; they also accused Federalists of
favoring Britain in order to promote aristocratic,
Adams was attacked by both the opposition Democratic-Republicans and a
group of so-called "High Federalists" aligned with Alexander Hamilton.
The Democratic-Republicans felt that the Adams foreign policy was too
favorable toward Britain; feared that the new army called up for the
Quasi-War would oppress the people; opposed new taxes to pay for war;
and attacked the Alien and Sedition Acts as violations of states'
rights and the Constitution. "High Federalists" considered Adams too
moderate and would have preferred the leadership of Alexander Hamilton
instead. Hamilton, in his third sabotage attempt towards Adams,
schemed to elect vice-presidential candidate Charles Cotesworth
Pinckney to the presidency. One of Hamilton's letters, a scathing
criticism of Adams that was fifty-four pages long, became public
when it came into the hands of a Democratic-Republican. It embarrassed
Adams and damaged Hamilton's efforts on behalf of Pinckney, not to
mention speeding Hamilton's own political decline.
Of course, the US did not invent smear campaigns. Marcus Tullius Cicero's brother published a book about electioneering after his brother's Consular election over Catiline, which was notable for smearing Catiline with killing his brother-in-law for money, trying to assassinate the Consuls a few years earlier, and raping a Vestal Virgin. So it has a long history.