Assuming we gain all the states we have now with new states going to the Union, yet some border states secede:
Since the Civil Rights Act does not apply to the South, there is no reason for it to stop being Democrat. But, while this is incredibly hard to predict, the South splinters into two parties. These parties were founded after a long Democratic dominance when the blacks in the South started to protest. People feeling the enforcement of Jim Crow laws and such was most important founded the Segregationist Party, those who felt preserving economic populism was most important found the Populist Party. In 2004, Populist candidate Mark Warner won a narrow victory over Segregationist David Duke. In 2008, the Segregationists nominated Fmr. Sen George Allen, more of a moderate, and choose Mississippi Governor Trent Lott as his running mate. The election played out like this:
Allen/Lott 113
Warner/Edwards 87
Segregationist Party reclaimed the Executive Mansion for the first time since the 1976 election when Strom Thurmond was president.