His Virginia victory left him a scant 10 delegates short of his rival, with 132 left to allocate for the night.
Her deputy campaign manager resigned, the second high-level Clinton departure in as many days.
The Illinois senator was gaining more than 60 percent of the Virginia vote, and Clinton, senator from New York, less than 40 percent.
The Associated Press made its call in the Maryland race based on surveys of voters as they left the polls.
Republican front-runner John McCain won both states, adding to his insurmountable lead in delegates for the GOP nomination.