Momentum has its own American political variant this morning: "Mitt-mentum."
That's because political observers are collectively stunned.
A week ago we were debating whether debates even mattered. A week ago Obama seemed in an unassailable position in the key battleground states.
And yet Mitt's momentum is now so significant that we're wondering what's going on. Are we overstating it? Have we missed something?
The best example of the changing dynamics of this race can be found in areas Democrats were taking for granted.
Pennsylvania and Michigan are states that were already in the President's column.
Republicans were spending no money there. Yet this morning both seem competitive.
So the question has become this: are we seeing a game-changing event that will allow Mitt Romney to win the Presidency or just a post-debate bounce that will fade over the next 27 days?
There are no answers at this stage.
Yes, we will have an avalanche of fresh polls in the coming days.
But many variables remain: Obama's performance in the remaining two debates, Joe Biden's success tomorrow night in the vice presidential match-up, unemployment figures due in three weeks, the state of the stock market, foreign policy issues - all can still play a role.
But the ultimate test - despite early voting - is going to be the ground game of the two parties on November 6th.
Who can mobilize their key supporters? Who can clinch the 8% of undecided, swing voters in Florida and Ohio?
Without a doubt, President Obama has been given a heck of a wake-up call.
You don't win the Presidency - or keep it for a second term - without needing stamina, hunger and resilience on an epic scale.
This is the greatest test of Obama's political skill and cunning since his great show-down with Hillary Clinton more than four years ago.
So as we enter the final stretch, let's see who can keep their nerve and win what is suddenly proving to be a highly competitive race.