To start with, nobody is forcing the clergy to have tax exempt status. It is their own decision to be tax exempt. If they don't like the conditions of the tax exemption they should give it up. Then they can say what ever they want to say and promote whichever politician they want to. The only reason they would not give it up is their lust for money which makes them put cash above free speech.
Secondly, as Christians they are violating the teachings which the Bible claims Jesus taught. In fact, at Matthew 17:24-27 the Bible claims that Jesus paid taxes himself! And of course there is the famous Bible story of Jesus teaching his followers to pay taxes when he allegedly said, "Render to Caesar what is Caesar's and to God what is God's" at Matthew 22:16-21. Christian clergy members today who are tax exempt are in violation of their Lord's own actions and teachings!
Senior Legal Counsel Erik Stanley of the Christian legal group Alliance Defense Fund, the group which organized the Christian clergy movement to "resisteth" the IRS, said, "The ADF is not trying to get politics into the pulpit; we want to get government out of the pulpit." Stanley must not be familiar with the Christian Biblical teaching at Romans 13:1-7 which makes it very clear the powers that be and government officials are in power because God put them their and they are "ordained of God." In fact, any one who believes in the Bible as the word of God, which includes the entire religious right, should be careful not to "resisteth" any of "the powers that be" or the Bible god will damn them according to 13:2. And specifically regarding taxes, this Bible gem states, "For this cause pay ye tribute also: for they are God's ministers, attending continually upon this very thing. Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour." This Christian Bible teaching to blindly obey your government in fear is why many of America's key revolutionaries were Deists and not Christians.
Richard Land of the Southern Baptist Convention seems to suffer from this same lack of understanding of his own "holy" book, the Bible. Land said that future Christian leaders will not be in the clergy, but instead will be like Glenn Beck, outside of the clergy due to the government restrictions clergy members who want to keep their tax exempt status are under.
Based on their own Bible, Christian clergy who want to shirk their taxes don't have a leg to stand on.