Hurricane Sandy - challenge for presidential rivals
The mettle of a president is tested not during a dull routine, but in a crisis. So the Hawaiian tsunami and the very real possibility of a massive storm hitting the east coast of America in the last full week before election day is changing the candidates' calculations.
This is what leadership is about. It may even be a vital test of temperament. Some politicians would be irritated by this interruption to their long-laid plans. Others would gladly seize it with both hands.
The network and cable news weather forecasters are having a ball predicting doom - there has not been a threat like this in 30 years, according to one. They may not be correct.
President Barack Obama has cancelled some campaigning, to be in the White House on Monday night, when the storm may hit the East Coast. Mitt Romney has pulled his plans to visit Virginia. Better safe than sorry.
The Republicans cancelled the first day of their convention because of the approach of a tropical storm. It turned out to be just a bit wet and windy, no worse than any average day in a British winter.
But the memory of Hurricane Katrina, and the suggestion that the damage it did to New Orleans was callously ignored, is lodged deep in their collective consciousness. They lost a day of their Convention out of an abundance of caution but it did them no harm.
This too might turn out to be nothing. But it could be a defining moment.
Mr Obama has to look like a leader in command, perhaps gently hinting of the benefit of government. He will have to curb his recent tendency towards mockery and jokes, which may be no bad thing anyway.
Mr Romney has a difficult task - to look presidential without adopting the duties. He can't flap around disaster zones getting in the way and looking like a political ambulance chaser. But he has to find the right tone.
My family and I live near where the storm will hit and we already know the havoc American weather can cause. It's pretty frightening. Millions of people will be very nervous for the next few days. So will the politicians.