Lets keep the issue clear here:
It's not a violation of the Constitution to preach about candidates in the pulpit.
It IS a violation of federal tax law to do so if you claim federal tax exempt status. (And perhaps campaign finance laws, too).
Recent comment from McGovern: "He (McGovern) ridiculed Bush for praying about going to war, and for Bush referring to God as his "Father in heaven"" /here: http://www.freerepublic.com/focus/f-news/1200860/posts