Final answer:
Thomas Jefferson was a leader and member of the Democratic-Republican Party, which was centered around states' rights and a decentralized federal government.
Step-by-step explanation:
Thomas Jefferson belonged to the Democratic-Republican Party, which he helped to establish in opposition to the Federalist Party. The party was founded in the early 1790s and advocated states' rights and an agrarian-based, decentralized federal government, which was in contrast to the Federalist Party that supported a strong centralized government and a more protective stance towards industry. Jefferson's leadership of the party led to his election as the third President of the United States in 1801.