I reject the premise that 10% or more Evangelicals will not vote for Romney, particularly in the face of Clinton. But, there will be few. My contention is that Romney’s crossover appeal - stemming mostly from his demonstrable family values - will dwarf that contingent.
Where is the evidence that Romney has any crossover appeal? Since Romney is generally the weakest of the major Republican candidates against the Democrats in hypothetical matchups, I really question Romney's crossover appeal. Many of the independents are secular and would be put off by Romney's devotion to Mormonism.
Don't count on Romney attracting evangelical votes because Clinton is the opponent. Clinton might not be the nominee. Also, evangelicals know that we have already survived eight years of Clintonian secularism. The difference between Romney and Clinton on a lot of issues isn't that great despite Mitt's claims to be a conservative. His flip-flopping and general softness on a lot of issues won't be reassuring to evangelicals. Finally, a lot of evangelicals are far more threatened by Mitt's Mormonism than they are Clinton's secularism.