Yes, Christian pastors must preach about the role of the Christian voter in society.
You cannot separate Christianity and society because Jesus Christ demanded making disciples of all nations in the Great Commission.
Christian obligation to stop baby-killing through abortion must be preached actively during Sunday sermons.
An important way to stop baby-killing through abortion is to vote against Democrat politicians during elections.
So, it is absolutely necessary for Christian pastors to preach against voting for Democrat politicians during Sunday sermons.