U.S. Senator
On December 1, 1814, Virginia's legislators elected Barbour (then 40) to succeed Richard Brent in the United States Senate. Although Barbour had previously opposed a national bank, President James Madison supported such, so Barbour became the Senate sponsor of a bill written by Secretary of the Treasury Alexander James Dallas, which authorized the National Bank with $50,000,000 in capital. It passed (although prior similar legislation failed).
Senator Barbour aligned with Senators John C. Calhoun and Henry Clay on internal improvements and slavery. Although his brother Philip P. Barbour served contemporaneously in the U.S. House, their stances and votes often differed. Senator Barbour proposed a committee on roads and canals, supported the Bonus Bill (authorizing spending the bonus from the national bank on internal improvements), and proposed a constitutional amendment to grant Congress the authority to appropriate money for internal improvements. Senator Barbour also opposed reducing the national army, supported a bill abolishing imprisonment for debts, and introduced the Navigation Act of 1818. That Act closed U.S. ports to any ships from British ports closed to U.S. ships. Barbour hoped this would encourage the British to open their ports. However, that effort failed. In 1823 a compromise led to the Elsewhere Act, which allowed for reciprocal trade.
Peers elected Barbour President pro tempore of the Senate in 1819. The 16th Congress, over which Barbour presided, adopted the Missouri Compromise on slavery. Barbour proposed combining the bill admitting Missouri (after he spoke in favor of allowing that state's voters to elect to support slavery) with the bill admitting Maine—both in an attempt to deny the Northern Senators an opportunity to gain four anti-slavery Senators. His speech may have foreshadowed the Southern position in the American Civil War after his death:
Sir, no portion of the Union has been more loyal than the South. Is this your reward for our loyalty? Sir, there is a point where resistance becomes a virtue and submission a crime. Our people are as brave as they are loyal. They can endure anything but insult. But the moment you pass that Rubicon, they will redeem their much abused character and throw back upon you your insolence and your aggression.
As Senator, Barbour sponsored a resolution giving an honorary sword to Colonel Richard Mentor Johnson of Kentucky for his efforts in the Battle of the Thames in 1813. Johnson and Barbour would become quick friends following Barbour's efforts. Later, Johnson promoted Barbour's appointment as Secretary of War under President John Quincy Adams. However, that association with Adams, whom the Senate narrowly elected over Andrew Jackson, would later devastate both Senator Clay's (and Barbour's) political careers.
Virginia legislators elected the Jacksonian Democrat John Randolph of Roanoke to succeed Senator Barbour in December 1825. Like Barbour, he would defend slavery, although a member of the American Colonization Society like Henry Clay. Unlike Barbour, he would later emancipate the people he enslaved upon his death. Randolph opposed the national bank and the Missouri Compromise of 1820 that Barbour had helped Clay pass.