Before I get to why political campaigning by nonprofits should be illegal, it's worth noting that this campaign promise doesn't seem to make much sense politically. According to a Lifeway poll of 2015, 79% of Americans disagreed with this statement: "I believe it is appropriate for pastors to publicly endorse candidates for public office during a church service." Only 25% of Americans with evangelical beliefs agreed with the statement. So it appears that the overwhelming majority of Americans, including a huge majority of evangelicals, don't like the idea of ministers endorsing political candidates anyway.
Besides that, the Johnson Amendment is pretty limited. Sure, pastors cannot, in their official capacity as ministers, endorse a candidate for public office. But ministers can get involved in political campaigns on their own time. They can place political bumper stickers on their cars, stick campaign signs on their property and otherwise use their own personal time and resources to promote (or attack) a candidate for public office. They just can't legally do so on the job.
Furthermore, the Baptist Joint Committee for Religious Liberty (BJC) is onto something when it says that allowing campaigning in churches does harm to the nature of church life. In a statement released BJC Executive Director Amanda Tyler, she said,
Inviting churches to intervene in campaigns with tax-deductible offerings would fundamentally change our houses of worship. It would usher our partisan divisions into the pews and harm the church's ability to provide refuge. To change the law would hinder the church's witness, threatening to turn pulpit prophets into political puppets.Okay, but why should it be illegal for tax exempt nonprofits, including churches, to be actively involved in promoting or attacking candidates for public office? Maybe most Americans don't want nonprofits doing political campaigning, maybe the limitations the law places on ministers are limited, maybe campaigning in churches can do harm to the church's witness. But why should it be illegal?
Simple. The Johnson Amendment assures that taxpayers do not subsidize partisan politicking. It also ensures that tax-exempt entities do not become a conduit for tax-exempt contributions to political candidates. Because it's not fair for taxpayers to subsidize partisan politicking and for tax-exempt organizations to serve as pipelines for tax-exempt donations to candidates, it should be illegal for tax-exempt nonprofits, including churches, to be actively involved in political campaigns.
Repealing the Johnson Amendment will literally require an act of Congress which appears to be unlikely. That's a good thing.