After millions of votes, Democrats outnumber GOP

(CNN) -- Early voting is changing campaign strategy and voter behavior like no other presidential race in history, experts say, as Sens. John McCain and Barack Obama make final cross-country pushes this weekend.

Both candidates are hoping to reap the lion's share of more than 23 million votes that have been cast nationwide. They're targeting states where polls remain open through the weekend -- and, in some cases, into Monday.

Calculations by CNN and other news organizations indicate that many, if not most, of the early votes in more than 30 states are being cast by registered Democrats, although it's unknown who voted for which candidate.

It would surprise me, but Republican voters in early voting states may simply be holding their ballots,
— Paul Gronke of the Early Voting Information Center said Friday.
But I wouldn’t be surprised to see the Republican race start to catch up a little bit.
— Paul Gronke of the Early Voting Information Center