A little history that has not yet played a role in the debates:
The founders admired the British system of government, and copied much of it, but they rejected the class society on which it was based. Hence the House of Commons becomes the House of Representatives; the House of Lords becomes the Senate, whose members do not obtain their seats via class lineage. The president was modeled on the king, with elections replacing the British reliance on royal dynastic succession.
The key thing that the founders knew—and we have all forgotten—is that numerous English or British kings were foreign born, indeed, were foreigners. The seventeenth-century kings James I and Charles I were born in Scotland (a foreign country to their subjects in England). William III, who in 1689 initiated the constitutional monarchy that existed at the time of the U.S. founding, was Dutch. The two early eighteenth-century kings, George I and George II, were born in Germany. This was a consequence of the rules of dynastic succession at a time when royal lineages crossed borders .
But the foreign connections of English and British kings were a constant source of political tension. English subjects distrusted the foreigners who occupied the throne. The alleged French sympathies of James II (not foreign born, but a Catholic and cousin of Louis XIV, who spent much of his early adult life abroad) led to the Glorious Revolution. In those days, it was not out of the question for a king to invite a foreign army to come to his aid by invading his own country. Parliament could not ban foreign kings but it did the best it could in the Act of Settlement of 1701, providing that if the Crown “shall hereafter come to any person, not being a native of this Kingdom of England, this nation [shall] not be obliged to engage in any war for the defence of any dominions or territories which do not belong to the Crown of England, without the consent of Parliament.” It also banned from the throne Catholics, and spouses of Catholics—namely, foreigners and native-born people thought to have foreign sympathies, like James II.
Could a foreigner born of an American citizen, possibly a foreign lord flush with cash, come to the United States and bribe his way to the presidency? Such a possibility would not have been far from the minds of the founders. The natural-born citizenship clause solved this problem in a stroke. It ensured that presidents, unlike British kings, would be free of foreign sympathies arising from foreign birth and connections. The founders required that representatives have been a U.S. citizen for seven years, and senators for nine years, before taking office. The “senatorial trust,” explains The Federalist Papers, “ought to be exercised by none who are not thoroughly weaned from the prepossessions and habits incident to foreign birth and education.” The presidency—an infinitely more important office than that of a senator—must be protected all that much more from foreign influence, they believed.
What does this mean for Cruz? Whatever the merits of his candidacy, it’s unlikely that he would, as president, call for an invasion by the Canadian army. But Cruz, whose campaign website is headed “RESTORE THE CONSTITUTION,” and who has criticized the Supreme Court for “redefining the meaning of common words,” is in a weak position to argue that natural born citizen is something it isn’t.