A pre-condition of serving in many political offices in the United States is pledging to uphold the US constitution. But the rub is that until many heretics and infidels were to allow the US Constitution to be amended regarding the Establishment Principle (which is very unlikely indeed), one would have to serve as magistrate without defending God's interests (as implied in one's pledge to abide by the non-Establishment Principle of the US Constitution). But is that a deal any reformed Christian can make?
Just to give an analogy- could a Christian father in good conscience sign a contract not to raise up his children in the fear of the Lord, until such time as heretical and infidel co-signers of the contract allowed? Or could a doctor sign a contract not to heal people, though practicing medicine? On the other hand, if one did sign such a pledge, but later in office sought to defend God's interests (before an amendment had been passed), would not the heathen be able rightly to accuse one of being a deceiver, liar and oath-breaker? Defending the first table of the Ten Commandments and Christ's church is as much a part of a magistrate's God-given duty as healing people is a doctor's duty. A magistrate could not rightly swear the duty away for a time. A civil magistrate must as a basic duty defend God’s interests in the first table of the law and man’s interests in the second table.
Rev. John Macleod of the Free Presbyterian Church of Scotland has correctly identified how contrary taking an oath to the US Constitution is in comparison to taking a sound oath, in this quote from the Free Presbyterian Magazine:
"Across the Atlantic, the Presbyterian Church of America in 1788 saw fit to alter The Westminster Confession of Faith , her subordinate standard, in order that its terms might be in conformity with the false view that Church and state are totally separate and that all religions are to be regarded as equal before the law. Little wonder that most of the cults which have spread plague-like throughout the nations originated in that land which by the First Amendment to its Constitution repudiated the Establishment Principle and laid down that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof". How far removed is the oath of office which the President of the United States takes on his inauguration from that which the framers of our National Covenant required of such as aspired to sit on the Scottish throne."
The Westminster Confession of Faith in its Chapter 22 correctly avers concerning oath-taking:
 EXO 20:7 Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain. JER 4:2 And thou shalt swear, The Lord liveth, in truth, in judgment, and in righteousness; and the nations shall bless themselves in him, and in him shall they glory.
 GEN 24:2 And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh: 3 And I will make thee swear by the Lord, the God of heaven, and the God of the earth, that thou shalt not take a wife unto my son of the daughters of the Canaanites, among whom I dwell. 5 And the servant said unto him, Peradventure the woman will not be willing to follow me unto this land: must I needs bring thy son again unto the land from whence thou camest? 6 And Abraham said unto him, Beware thou that thou bring not my son thither again. 8 And if the woman will not be willing to follow thee, then thou shalt be clear from this my oath: only bring not my son thither again. 9 And the servant put his hand under the thigh of Abraham his master, and sware to him concerning that matter.
 NUM 5:19 And the priest shall charge her by an oath, and say unto the woman, If no man have lain with thee, and if thou hast not gone aside to uncleanness with another instead of thy husband, be thou free from this bitter water that causeth the curse. 21 Then the priest shall charge the woman with an oath of cursing, and the priest shall say unto the woman, The Lord make thee a curse and an oath among thy people, when the Lord doth make thy thigh to rot, and thy belly to swell. NEH 5:12 Then said they, We will restore them, and will require nothing of them; so will we do as thou sayest. Then I called the priests, and took an oath of them, that they should do according to this promise. EXO 22:7 If a man shall deliver unto his neighbour money or stuff to keep, and it be stolen out of the man's house; if the thief be found, let him pay double. 8 If the thief be not found, then the master of the house shall be brought unto the judges, to see whether he have put his hand unto his neighbour's goods. 9 For all manner of trespass, whether it be for ox, for ass, for sheep, for raiment, or for any manner of lost thing, which another challengeth to be his, the cause of both parties shall come before the judges; and whom the judges shall condemn, he shall pay double unto his neighbour. 10 If a man deliver unto his neighbour an ass, or an ox, or a sheep, or any beast, to keep; and it die, or be hurt, or driven away, no man seeing it: 11 Then shall an oath of the Lord be between them both, that he hath not put his hand unto his neighbour's goods; and the owner of it shall accept thereof, and he shall not make it good.”
And the Testimony of the RPCNA has these helpful words on oath-taking and voting:
“16. It is sinful for a Christian to take an oath which compromises his supreme allegiance to Jesus Christ. It is also sinful to vote for officials who are required to take an oath which a Christian himself could not take in good conscience. Voting involves the voter in responsibility for any act required of the official as a condition of holding his office. Deut.10:20; Isa.45:22-23; 2 John 1:11; I Tim.5:22.
So it is morally dubious for a reformed Christian magistrate (or one who seeks that office) to take such an oath to uphold the US Constitution with its non-Establishment clause and its prohibition of religious tests. And it is morally dubious to vote for someone who would take such an oath.