For decades, Southern Democrats could count on winning local and statewide offices, even though the voters in their states would often withhold support for the Democrat in presidential races. No longer.

Despite efforts to distance themselves from President Obama, none of the Democratic Senate candidates in the South outdid his 2012 results on Tuesday. Democrats lost Senate races, sometimes by wide margins, in Kentucky, Georgia, Louisiana, Arkansas and North Carolina, most of which were thought to be competitive for much of the year. They nearly lost in Virginia, where they were thought to be heavy favorites and where The New York Times has not yet projected a winner.

The inability of Southern Democrats to run well ahead of a deeply unpopular Mr. Obama raises questions about how an increasingly urban and culturally liberal national Democratic Party can compete in the staunchly conservative South. It raises serious doubts about whether a future Democratic presidential candidate, like Hillary Clinton, can count on faring better among Southern white voters than President Obama, as many political analysts have assumed she might.

The Democrats running in the South this election season were not weak candidates. They had distinguished surnames, the benefits of incumbency, the occasional conservative position and in some cases flawed opponents. They were often running in the states where Southern Democrats had the best records of outperforming the national party. Black turnout was not low, either, nearly reaching the same proportion of the electorate in North Carolina, Louisiana and Georgia as in 2012.