The simplest answer is that US increases in demand can be met, more or
less, with expansions in energy efficiency.

The real issue is that we have many coal fired power plants planned for
the future, and these are way to carbon intensive to consider in a
greenhouse constrined world. (Coal plants would all have to be carbon
capture ready, at an added cost of ~ 3 cents/kWh based on current
estimates,
but this is NOT currently planned). With coal at ~ 3 – 5 cents/kWh
already, this makes coal a very expensive proposition relative to
nuclear (and, actually, several other renewables), adding this 3
cents/kWh
kicker makes it an expensive deal, even before the other negtives of
coal.

So, to answer this, a simple, but a bit long response you could send
would be:

Making the response pretty simple (and overlooking lots of
transition issues)

The US used ~ 3 trillion kilowatt hours in 2005, dividing by 365
(assuming
the plants are always on) = a capacity of 10^10 kW

Right now:

coal = 50% of US electricity
nuclear = 20% of US electricity

I'm ignoring natural gas, as that is about 50% less carbon intensive
than coal), but it can be estimated from this and my coal numbers below

Lets assume:
each nuclear, or coal plant comples is about 1 GW (1000 MW, or, a
million
kilowatt hours. Lets assume the plants each run all the time (actually,
they run ~ 90% of the time if nuclear, and 70 – 80% of coal, but
ignoring
that).

So, each new plant, at 1 million kilowatt hours per plant per year, tips
the
coal vs nuclear balance by:

(1 million kW plant = 10^6 kW)/(10 billion kW total US capacity = 10^10
kW)

= 10^(-4), so on a percentage basis (x 100)

this is 10^(-2), or a tenth of percent of total US electricity per
plant decision.