In the fall of 1826, Polk returned to Washington from Congress' summer recess, this time bringing his wife along. They found a small suite in a private house–just one bedroom and a parlor–and Sarah Childress Polk set about turning it into a suitable living arrangement for them. Polk bought her a piano as a way of apologizing for his long hours at work. But before she could learn to play the new instrument, she found herself swept up in the social world of Washington politics. Her shining personality helped boost the up-and-coming political couple's standing on Washington's social ladder and they soon found themselves swamped in invitations to dinners, receptions, balls and breakfasts. Sarah, however, managed to devise a way to ensure that business meetings never kept her husband from church on Sundays. As the church bells called, she would waltz into her husband's strategy meetings in the parlor with his hat and cane and say, "If we don't hurry, we'll all be late for church." After a few Sundays spent with the Polks at the First Presbyterian Church, most of his friends learned not to call on the Polks before noon on Sundays.

The Polks soon settled into a comfortable commuting relationship. For the fourteen years that James K. Polk held his seat in Congress, he faced only occasional opposition in the elections, leaving him free to spend his recesses traveling. The Polks made each commute back and forth from Tennessee to Washington a leisurely vacation, traveling sometimes by horseback, sometimes by stage and even sometimes by steamboat. Once they spent a recess traveling through New York, ending at Niagara Falls.

During Polk's first three years, he became one of Andrew Jackson's most vocal supporters in the House and a constant thorn in the side of President John Quincy Adams–who had been elected in a "corrupt bargain" in 1824. Jackson and his supporters never stopped campaigning in the intervening years, for they believed that Jackson had legitimately won the election. Polk played a key role in the campaign by opposing internal improvements and fighting for tariff reductions. He became respected for his attention to and knowledge of details and his fighting spirit brought him to the forefront of the ranks of Jacksonians. Nonetheless, members on both sides of the aisle respected him for his courtesy and his even temper in debates. Although he lacked the charisma to develop a cause on his own and attract followers, Polk's drive and organization made him the perfect person to pursue a set goal or pass a predetermined cause.

In 1828, Jackson ascended to the presidency, after overwhelmingly winning both the popular vote and the Electoral College. Jackson's win changed Washington overnight, as "Old Hickory" assumed office among one of the rowdiest inaugurations ever recorded. Polk, who had spent his first years in Congress vigorously opposing anything the president wanted, now–with a president of his own party in power–quickly changed his mind and saw opposition to the president's wishes a gross miscarriage of democracy.

One of Polk's first major battles was with another of Tennessee's congressmen, the legendary frontiersman Davy Crockett. Crockett had developed a severe aversion to Jackson and his supporters and he blocked a bill by Polk that would have established Land Grant Schools throughout the state. Crockett, an uneducated man himself, found little use for education and said that only rich Tennesseans would get to benefit. Despite Polk's best efforts, he could not pass the bill during Crockett's time in Congress.

After the land grant battle, there arose a political maelstrom unlike Washington had seen in years–the so-called "Peggy Eaton affair." Detractors (chief among them Jackson's heir-apparent, John C. Calhoun) whispered that Eaton, the wife of the secretary of state, had taken several lovers before she married her current husband. As the rumors spread, the Eatons found themselves shut out of Washington social life. Jackson, who was personally insulted by the mess, shunned meetings with anyone who ostracized the Eatons–which amounted to most of Washington. Polk and his wife, though, remained quiet on the issue and Sarah Polk often went out of her way to greet the Eatons in public, much to Jackson's pleasure. Despite Congressional attempts to force the Eatons from office, Jackson held strong. The secretary of state, Martin Van Buren, seized the opportunity to draw a stark contrast between his gracious actions and Calhoun's whispers. Then, Van Buren let slip that Calhoun had once supported a presidential reprimand of Jackson's military campaign in Florida just a decade before. In one quick moment, Calhoun found himself shut out of the Jacksonian party and Van Buren found himself the new heir-apparent. Polk, too, saw his prospects rise in the new political landscape. A simple congressman, Polk was becoming one of the party's most important figures.