Do you think that all those who fought for the South in the Civil War were “pro slavery”? You might consider the importance of FREEDOM of choice (even if it is a wrong choice). You also might consider the harm done to the political environment when you make an issue so divisive as this the determining factor of an election. The result will be a loss of conservative influence.
The freedom of the individual ends when it infringes on the freedom of another individual- especially when that freedom is the freedom to live. That is why both abortion as was slavery, are the most vital issues as they deal with the most fundamental freedom- that of the individual to live his/her own life.