The South wins Gettysburg, and the Civil War lasts longer. In desperation, the North orders all of its generals to treat captured territory in the same way as Sherman did. The war lasts a few years longer, with a Confederate insurgency fighting until the very end. With this, former slaves make up a majority in many areas. With few Southerners able or willing to be convinced to love the United States again, Congressional Reconstruction lasts longer, leading to full equality of blacks in the South much faster. This leads to a mostly black South, as well as fewer black migrants to the industrial cities in the North. The much poorer, largely still discriminated against black communities in the South therefore vote liberal, or even Socialist. This creates a conservative backlash in the North.
Another, probably better option would be to keep New England more sparsely populated by butterflying away the Irish Potato Famine. Most immigrants will arrive in New York, but few will reach Boston. Without a large, poor populace, the Democrats will struggle to gain control of the North. You would therefore see alot of elitist conservatives, generally more socially liberal while economically corporatist and fiscally very conservative.