Who ran against Thomas Jefferson in the election of 1800?

Jefferson's opponent in the presidential election of 1800 was the sitting president, John Adams. The election was hugely significant in that it was the first truly partisan contest in American political history. George Washington had recently passed away, and without a galvanizing figurehead to lead the United States, the country split along party lines. President Adams, a Federalist, was deeply unpopular. He had presided over a growing centralization of political power, one that earned him the undying enmity of his Republican opponents, who accused him of being a tyrant.
The Republicans, unofficially led by Adams's Vice President, Thomas Jefferson, saw themselves as heirs to a tradition of radical republican liberty, the most precious bequest of the colonists' victory over the British. To them, the Federalists constituted a threat to Americans' hard-won freedom. They held that ultimate sovereignty resided with the states, in keeping with the radical decentralization of power under the Articles of Confederation. The growing power of the federal government under Adams represented a complete antithesis of the "Spirit of '76." The passing of the draconian Alien and Sedition Acts appeared to confirm Republicans' suspicions.
The Federalists, for their part, looked upon the Republicans as dangerous dreamers, utopians infected with unworkable ideas. They had a disturbing obsession with the French Revolution, despite the constant threat of French ships towards American vessels. Federalists believed Republicans' inflexible approach to states' rights and their bias towards farmers would hold back the development of the American economy.
The election campaign was marked by extraordinary levels of bitterness and personal abuse. Both sides resorted to smears and outright slander against each other: Jefferson was deemed an atheist, a traitor in the pay of France; Adams allegedly wanted to take the place of George III and make himself king. Yet, when the dust had settled, and the votes had been counted, some measure of stability returned. Jefferson's gracious inaugural address took some of the heat out of a tense political situation. But the precedent of partisanship had been established, and American politics would never be the same again.