From a Christian stand-point, calling 70+% of the U.S. population Christian is just untrue.
A more accurate statistic would be Protestant Christian pastors who support evolution, but not ID or Creationism.
From the same page,
Beliefs among conservative Christians:
In 1999-NOV, Focus on the Family, a Fundamentalist Christian agency, concluded a poll of their web site visitors concerning their beliefs about creation and evolution. Results were:
God created the universe, but I don't know when: 46%
God created the universe thousands of years ago: 43%
God created the universe billions of years ago: 10%
Life came into being and evolved on its own: 1%
I don't have a clue: 0.4% 6
You are creating a false dichotomy by limiting the definition of Christian to those that do not accept evolution. The acceptance of Evolution is not restricted to atheists (or at least to non-Christians).
You can't put forward a convincing argument by redefining terms.