In his fine novels of an alternative universe, starting with "The Probability Broach," science fiction author L. Neil Smith asks what America would have been like if the Declaration of Independence had contained just one additional word -- if Mr. Jefferson had thought to declare that governments derive their just powers from "the unanimous consent of the governed."
Interestingly enough, there's a real-world example that corresponds pretty closely to this. In the Polish Sejm, No proposal could become law, and no decision was binding, unless it received the full assent of all those persons who were competent to consider it. A single voice of dissent was equivalent to total rejection. Majority voting was consciously rejected. ... Three lines of reasoning can be discerned. One argument, in a state where the executive arm depended on the voluntary support of all its citizens, was purely practical. Laws and decisions which were passed in the face of opposition could not have been properly enforced. The second was based on the consideration that the prospect of chaos might concentrate men's minds on harmony. The third derived from the somewhat naïve belief that institutions which are less than perfect are not worth keeping anyhow.
It [the principle of unanimity] was responsible for two constitutional practices, the Confederation, and the Liberum Veto, which made the [Polish] Republic famous throughout Europe.
The Confederation was an expression of the citizen's fundamental right to resist. It was an armed league of men sworn to pursue their grievance until justice was obtained. Confederations were formed in 1302, 1382-4, 1439, 1560s, 1573, 1656, 1672, 1704, 1715, 1733, 1767, 1768 and 1792.
In effect, the Confederation was a legalized form of civil war, and no one thought it unusual.
The Liberum Veto came into flower rather later than the Confederations, though it too was grown from very ancient roots. It was a device whereby any single member could halt the proceedings of the Sejm by the simple expression of dissent.
Usually vetos would be handled within the Sejm, with the Marshal questioning the dissenter, finding out what the problem was, and working out a compromise. Later in Polish history, vetos probably helped lead to the downfall of the Republic and the Partition.
One veto in 1580 blocked all taxation for that year.
In 1652, six weeks into the session, the agenda was still full of unfinished business, and the Marshal rose to announce a prolongation. A single voice was hears: 'Nie pozwalam.' (I do not allow it.) All the work of the session was null and void. The dissenter (Jan Sicinski) had acted on the orders of Janusz Radziwill, who ended up giving the Swedes a protectorate over the Duchy of Lithuania.
In 1666 the Liberum Veto was invoked in the middle of the session. In 1668 it was invoked on the opening day, before the debates had begun. In the reign of Augustus II (1697-33), 11 out of 20 sessions of the Sejm were broken. Under Augustus III (1733-63), only one Sejm was able to pass any legislation at all.
This allowed the Russians to put Russian troops in Warsaw, and effectively take over the country. It took until 1791 for the Poles to take back control of their own country, and only then because the Russians were preoccupied with the Turkish War. In 1793 the King was persuaded to retract the reforms and sign the Second Partition.