There has been an increasing number of people shifting to calling themselves 'independents'. No one can win on the National front anymore without holding a significant portion of the middle. Bush needed it. Obama needed it.

So what's with the 'traditional' GOP and Dem base? Who is it anymore and what percent?

I suggest that in each case the 'base' is about 20% on each side. The hard right religious bunch and the loony liberals. I don't believe either HARDCORE base TRULY has more than 20% of the voters. The other 60% are in various states of 'somewhere around the middle'. Call it another 10% on each side that are 'softcore' bases, strong leaners but not rabid frothing at the mouth entrenched in an all or nothing do or die the other side is the ultimate evil mentality.

20% hard right
15% soft right
30% up for grabs
15% soft left
20% hard left

So does it make sense for politicians on the national level to continue catering to the 'base'? The real fight is for that center 30%. If you catch a lot of it you can make up for some loss in the base but if you piss off the middle (go too extreme) to keep the base you can't possibly win.

What's the worst that can happen if they appeal more to the middle?

The worst is the base stays home. They sure as shit ain't going to go to the polls and vote FOR the devil.

How likely is that (staying home) given the mentality of the other side is pure evil and cannot be allowed to win? I'd say no more than 5% maybe 10% if times are good and they are really apathetic.

The traditional power of the bases IMO is broken. The power is all in the middle and the middle doesn't like EITHER extreme. Sticking too far to one side and alienating the middle will lose, every time. It didn't used to be that way. But it is now.

A lot of folks think the GOP will come back because they always have. I think the equation has changed and neither party can win now by holding to the base.

The bases are not where it's decided anymore on the national level. The GOP sticking to it's base is WHY it's not making any headway. Yeah it's worked before.

But not anymore. And were situations reversed, it wouldn't work for the Dems either.