How did Obama get elected and re-elected? No true Christian would have voted for a President were abortion would be in play. "That is above my paygrade", would not have played.
Main line churches are now debating or outright accepting homosexuality. Pornography, lust, and sensuality are rampant and out of control.
We still think it is unconstitutional to promote Christian ethics at the state and local levels. We can't even get that right.
So I don't see greater than 20%. There are a lot of social conservatives; but don't confuse them with Christians. A 50% demographic of real Christians would result in a moral transformation of this country. As it stands, it's not happening.
....Yet there are glimmers of hope. The major voting outburst this past Tuesday shows that Americans and in a special way, those who are Christian have had enough.
The seeds have been planted for that future transformation.
I think your 20% is being generous. I think 10% is far more likely.
America still clings to its Christian roots through tradition so appears to be more Christian than it actually is. But that facade is getting stripped away with each passing day.