What's your source on that statistic? I'm a member of the church where BOTH Cal Thomas and his good friend Bob Beckel attend. The church is firmly evangelical. Our pastor never mentions politics from the pulpit (though he will mention abortion as an evil). Other than Beckel, I know of a few political liberals there, but the vast majority, I'm certain, are conservative--primarily because, I think, they know they cannot in good conscience be a part of "The Abortion Party."
I simply don't believe that 50% of those who identify as "evangelical" and regularly attend church are liberal.
Here's a bit of political advice. Be careful who you label as conservative, because conservatism has other factors then simply social issues, and if you look closely, you will even see the splits in that sector.
Take blacks and Hispanic's for example. Much effort has been made in the study of their voting habits, and both groups define themselves as largely pro life and for school choice, but on fiscal and governmental issues like crime, health care, global warming and others, they are decidedly liberal and vote Democrat in most all elections with only a few exceptions like Cubans.
The sad truth about your evangelical group is that they often vote democrat or they don't vote at all if not urged to do so by some social issue that concerns them directly and this has been problematic in the Republican coalition in the past, in the present and in the future.
If you care to look, you will find that 50% or more of the self described evangelicals consistently vote democrat, and thems the facts, whether surprising to you or not.
The exit polls, so often cited by pundits, are never asking democrats if they are evangelicals. They only pose that question to repubs.
The best way and most fair way to get some idea of what the democrat/republican voter spit actually is, would be to estimate it like the general population.
That's what I am doing essentially. The only poller who really delves into this stuff is Zogby, and you will find a lot on his site, going back to 2000.
The trend however into 2008 is showing a much larger democrat influence in the Evangelical community, and some think it will reach 60% or more!
Granted, that is a lot of moral relativism, but then people rarely look that deep when they think the planet is about to boil their children as the warming alarmists have decried.
Check it out, make your own assumptions, but Obama got a lot of them in the primary and will have them in the general. Whether they break over 50% is a guess at this point, but in 2004 they got about 42% and before that, only 15%. The trend says they will break 50.