(CNN) -- In a presidential campaign laden with historic twists, the biggest shock may be yet to come: With three weeks to go, the first of two remaining debates tonight, and most Americans yet to cast their ballots, the final surprise may be that there are no surprises left. This campaign for president may be over. There is a good chance that the Denver debate was the watershed that carries this election to its conclusion. If so, Barack Obama has only himself to blame. Yes, the polls show a very close race, but the momentum has belonged to Romney since his superior showing in the first debate.
@highlight
Alex Castellanos: What if there are no more surprises in the presidential campaign?
@highlight
He says the momentum since the first debate has been all in favor of Romney
@highlight
It may be too late for Obama to make up for his debate weakness, Castellanos says
@highlight
Castellanos: Both candidates have shown they can handle town hall format
All that changed two weeks ago in the pure @placeholder air.