Unless one of them has a health problem, I don't see it happening, and it's useless to speculate on such small odds.

They both won their primaries fair and square.  McCain is a compromise candidate between the fiscal and the social conservatives; choosing someone else would alienate half their base and guarantee a loss.

On the other side, there is so much invested in Obama that it would rip apart the Democratic party if they got to the convention and somehow, some way, the delegates chose someone else.  It would alienate younger and black voters for a full generation, and make us look like idiots.

In short: No.

let it go.

In a rational universe, yes.  This is a rational universe and thus I can conceive of the possibility, but by the same token, I could conceive of zombies.  Unless McCain should suddenly fall ill (or Obama for that matter), it's unlikely.  I do secretly wonder, though, if the Republicans are prepared to field another candidate if McCain should fall ill.  Who would it be?  Huckster?

Theoretically, sure.

Theoretically, though, two meteors could fall at exactly the same time on the locations where they're campaigning.

However, if someone's paying out any less than 1,000,000-1 on John McCain or Barack Obama not being the nominee, don't take it.

Next question.

'Could' is the key word here.
Could it happen?
Sure.
Theoretically, there are very few things that fall outside the realm of 'it could happen'.

However, 'will' is far more definitive.
WILL it happen?
Nope.

Come November we will be voting in a contest between Barack Obama (D) and John McCain (R).

I, for one, am voting for the one with the (D) next to his name.  ;)
Something tells me that the majority of my fellow Americans will be doing the same.
I look forward to President Barack Obama.
:)

If the polls show an insurmountable lead of Obama over McCain before the republicon convention, then I could see the GOP nominating a dark horse.  Perhaps even Jeb Bush.  I'll bet there's not a republicon who would object.

http://www.huffingtonpost.com/steve-rose nbaum/when-mccain-drops-out_b_107236.htm l

There's a dimension imaginable in which McCain, clearly getting trounced in every metric deep into September, drops out "for health reasons."

His VP choice (whoever that ends up being) steps in as the party nominee, and the media fixate on it throughout October. This VP might well be a lesser-known quantity with enough marketable qualities for the media to lap up. He offers vague palliatives and standard conservative bromides designed to both confuse and assure the base, while presenting a less-antiquated alternative to Obama.

Actually, it wouldn't surprise me at all. Tossing out a new, last-minute product and hoping it sells might be the Republicans' only chance this time. It's their Hail Mary, if it comes to that.

Obama, on the other hand, is our solid nominee, and will be the next president.

I do not think McCain could lose his and the only way Obama loses his is if another Reverend Wright like Scandal emerges that he cannot cover with a speech.

Only a serious health issue or a Spitzer-sized scandal will stop it at this point.