In the March 19 letter to the editor titled, "Church should 'keep its paws out of government,'" the writer has turned almost inside out what is commonly called the "separation of church and state" at least part of it. I have no intention of standing up for the Mormon church. I just believe this is the wrong legal issue to invoke.
Let me quote the text from the First Amendment since it is short: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof...."
The first half doesn't apply here because there would have to be something such as a law requiring Mormon baptism in order to be a citizen of the state. The second part is quite clear, individual members of any church have the right to practice their religion up to and including making their voices heard on what they deem to be religious matters taken up by the state. If you could stop this kind of individual speech, you would be prohibiting their right to the free exercise of their religion.
Now, it's quite a different matter, in my opinion, for the church to spend money to change votes. In that case, they should have to pay taxes like all other groups should (I say "should" because as far as I know there is no politically active religion paying taxes) have to do if they are spending money to influence votes. I'm confident the U.S. Constitution is silent on that matter but you would likely find relevant tax laws if you looked it up. This is controversial, but I believe it is correct.