This district had historically been a Democratic Party stronghold. Antipathy to the Republican Party had its origins in the American Civil War and the infamous General Order 11. The Union Army ordered evacuation of the county in an attempt to reduce support for and the power of bushwhacker guerrillas. After the Civil War, there was disfranchisement of white males (mostly Democrats) who had been active for the Confederacy until they took loyalty oaths, or until 1870. The area was filled with conflict between Missouri's Radicals, who joined the Republicans, and Conservatives, who were Democrats. By 1880 former secessionists dominated Missouri's congressional delegation and state legislature.
Gradually this area developed a character similar to Yellow Dog Democrat districts in the South. Until 2010, only one Republican had been elected here since the Great Depression, and only for one term. Bill Clinton, former governor of Arkansas, carried this district by a lopsided margin in 1992 and carried it again by a smaller margin in 1996.
However, several demographic trends have converged to erode the Democratic base in this district. First, as the New York Times election maps show, the predominantly rural counties lining the Missouri River have sharply trended Republican between the 2000 Senate election and the 2006 election, following trends across the South. Secondly, population losses in Kansas City resulted in the 4th gradually losing much of its share of heavily Democratic Jackson County to the Kansas City-based 5th district. Until the 1970s, the district stretched as far as Independence. To compensate for this, large portions of heavily Republican Southwest Missouri were reassigned from the neighboring 7th district. The result of these trends resulted in a dramatic collapse of Democratic support in the district. Al Gore, John Kerry, and Barack Obama won less than 40% of the vote here. In the 2010 general election, Hartzler defeated 34-year Democratic incumbent Ike Skelton.