With 91 percent of the votes counted, Van Hollen was leading by more than 7,000 votes, and Morella conceded the election -- ending her 16-year reign in the U.S. House of Representatives.
"We felt that there was no way she was going to make that margin up," said Tony Caligieri, Morella's campaign manager.
Though Morella's defeat marked a gain in a House seat for the Democrats, late exit polls showed that the likelihood of the Democrats taking control of the House was slim.
Van Hollen celebrated his victory at the Indian Spring Country Club in Silver Spring.
"He's ecstatic," said Winston Sale, who worked on Van Hollen's campaign. "Ecstatic and relieved."
As for Morella, Caligieri said she was satisfied with the campaign and was looking forward to relaxing.
"She was going to her beach house, get a stack of books and read for a week," he said.