The same demographic shifts that made it possible for Mr. Obama to carry Virginia twice after 10 consecutive Democratic losses have raised the water level for the party’s statewide candidates, too.

The swelling number of Virginia’s suburban, college-educated, moderate, minority voters — black, Hispanic and Asian — is hardly the only factor shaping the race. It is certainly not the most conspicuous, as scandal swirls around Gov. Bob McDonnell, the departing Republican, and buffets Mr. Cuccinelli at the same time.

But continuing shifts in the electorate increase the odds that Mr. McAuliffe can take advantage. “Demographic changes have leveled the playing field for Democrats in off-year elections, and have given us a huge pool of new voters to target,” said Geoff Garin, the Democratic pollster who advises Mr. McAuliffe.

Democrats elsewhere hope that shifts in the national electorate give them a similar lift in Congressional elections next year. If they get that boost, it could limit the damage presidents almost invariably suffer in midterm contests as Republicans push to hold the House and recapture control of the Senate.