With Virginia, the total number of states signed on to the interstate compact is now 18, plus the District of Columbia, for a total of 222 electoral votes.

The compact doesn't go into effect, though, until there are enough states signed up to reach the required 270 electoral votes to elect a president.

"This [effort] started 20 years ago and it's been slow and steady … constant forward momentum across these 20 years," said Alyssa Cass, a strategist for the National Popular Vote Project and a Democratic consultant. "Bills have been introduced in almost every state, most passed in a bipartisan way. This is on the 5-yard line of making this a reality."