Florence is now expected to pass over much of SC, with storm force winds beginning Thursday and hurricane force winds on Friday, said John Quagliariello with the National Weather Service in Columbia.
Duke Energy has around 4 million customers in North and SC, and says it could take several weeks to restore the electricity.
David Fountain, president of Duke Energy North Carolina, emphasized the potential implications of hurricane, which is now a category 4.
Life-threatening storm surges of up to 13 feet (3.9 meters) were also forecast in some areas along with the possibility of tornadoes in North Carolina.
"Get out of its way, don't play games with it, it's a big one, maybe as big as they've seen", Trump said.
By 8 p.m. ET on Wednesday, the storm was about 335 miles southeast of Wilmington, North Carolina.
Across several eastern states, residents scrambled to leave as the Category 4 storm, with winds of 225kmh, approached.
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Georgia Governor Nathan Deal, concerned the storm would bring its devastation south, issued an emergency declaration for all 159 counties in his state.
Governor Deal said yesterday: "The state is mobilising all available resources to ensure public safety ahead of Hurricane Florence".
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Updated NHC forecasts showed the storm lingering near the coast of the Carolinas, carrying days of heavy rains that could bring intense inland flooding from SC to Virginia.
People fleeing coastal North and SC clogged highways Wednesday as Florence bore down on the coast for a direct hit in a low-lying region dense with beachfront vacation homes. "There's nobody left to work".
In a trailer park outside Wilmington, Alondra Espinoza was preparing to leave with her two young children.
"I've been through hurricanes before but never with kids", she said.
More than 300,000 people have already evacuated from the coast, McMaster said, and traffic on the highway was moving steadily on Wednesday.