I parsed Kmiec in TIME.
Take a look at these pieces by Card. Ruini and Card. Scola on secularism. They are posted by Sandro Magister.
Here is how they see the Church’s role in the public sphere: if it were silent, for example, about life and death, "it would not contribute to the good of all"
Another great quote from an Eastern bishop:
"The West must decide to understand what influence faith has in the public life of its citizens, it cannot dismiss the problem."
Some people, I think Kmiec included, presume that the Church has been effectively defeated in the public square and it she is vociferous in defending certain doctrines, she will be completely marginalized. Therefore churchmen shouldn’t "interfere" too much in public affairs. Otherwise, just chime in on the "acceptable" issues, avoiding those which are too thorny.
These two pieces might also be interesting for the discussion of religious liberty: how the Church with her clear conviction and doctrine interacts with the state.