I agree. Mixing politics and religion means those who disagree with your politics will assume they also must oppose Christianity, and vice-versa.
Your beliefs about God will invariably shape your world view, but preaching while running for President can only end in mud getting tossed at Jesus.
The First Amendment requires that expressions of faith be neither prohibited nor preferred. We should not banish religion from the public square, but should guarantee access to all voices and views. We should share and debate our faith, but never seek to impose it. When discussing faith and politics, we should honor the "candid" in candidate - I have much more respect for an honest atheist than a disingenuous believer.
My faith is my life - it defines me. My faith doesn't influence my decisions, it drives them. For example, when it comes to the environment, I believe in being a good steward of the earth. I don't separate my faith from my personal and professional lives.
Real faith makes us humble and mindful, not of the faults of others, but of our own. It makes us less judgmental, as we see others with the same frailties we have. Faith gives us strength in the face of injustice and motivates us to do our best for "the least of us."
Our nation was birthed in a spirit of faith - not a prescriptive one telling us whether to believe, but one acknowledging that a providence pervades our world.