“It is substantially true, that virtue or morality is a necessary spring of popular government. The rule, indeed, extends with more or less force to every species of free government.”

Letter to John Taylor (28 May 1816) ME 15:23 http://www.britannica.com/presidents/article-9116907
1810s
Context: We may say with truth and meaning that governments are more or less republican, as they have more or less of the element of popular election and control in their composition; and believing, as I do, that the mass of the citizens is the safest depository of their own rights, and especially, that the evils flowing from the duperies of the people are less injurious than those from the egoism of their agents, I am a friend to that composition of government which has in it the most of this ingredient. And I sincerely believe, with you, that banking establishments are more dangerous than standing armies; and that the principle of spending money to be paid by posterity, under the name of funding, is but swindling futurity on a large scale.

Speech to the National Convention, (5 February 1794), as quoted in The Bolshevik Revolution, 1917-1923, Vol. 1 (1951) by Edward Hallett Carr, p. 154
Variant translations:
The attribute of popular government in a revolution is at one and the same time virtue and terror. Terror without virtue is fatal; virtue without terror is impotent. The terror is nothing but justice, prompt, severe, inflexible; it is thus an emanation of virtue.
As quoted in Red Star Over Southern Africa (1988) by Morgan Norval, p. xvi
If the mainspring of popular government in peace time is virtue, its resource during a revolution is at one and the same time virtue and terror; virtue, without which terror is merely terrible; terror, without which virtue is simply powerless.
As quoted in Rousseau, Robespierre and English Romanticism (1999) by Gregory Dart
Terror is nothing other than justice, prompt, severe, inflexible; it is therefore an emanation of virtue; it is not so much a special principle as it is a consequence of the general principle of democracy applied to our country's most urgent needs.
Original French: La terreur n'est autre chose que la justice prompte, sévère, inflexible; elle est donc une émanation de la vertu ; elle est moins un principe particulier, qu’une conséquence du principe général de la démocratie, appliqué aux plus pressants besoins de la patrie.
From Sur les principes de morale politique http://www.royet.org/nea1789-1794/archives/discours/robespierre_principes_morale_politique_05_02_94.htm

Madison's notes (31 May 1787) http://avalon.law.yale.edu/18th_century/debates_531.asp
1780s, The Debates in the Federal Convention (1787)
Context: Mr. MADISON considered the popular election of one branch of the National Legislature as essential to every plan of free Government. He observed that in some of the States one branch of the Legislature was composed of men already removed from the people by an intervening body of electors. That if the first branch of the general legislature should be elected by the State Legislatures, the second branch elected by the first-the Executive by the second together with the first; and other appointments again made for subordinate purposes by the Executive, the people would be lost sight of altogether; and the necessary sympathy between them and their rulers and officers, too little felt. He was an advocate for the policy of refining the popular appointments by successive filtrations, but though it might be pushed too far. He wished the expedient to be resorted to only in the appointment of the second branch of the Legislature, and in the Executive & judiciary branches of the Government. He thought too that the great fabric to be raised would be more stable and durable, if it should rest on the solid foundation of the people themselves, than if it should stand merely on the pillars of the Legislatures.

From The Fair Trade Fraud (St. Martin's Press, 1991) http://www.jimbovard.com/Epigrams%20page%20Fair%20Trade%20Fraud.htm

Campaign speech at Madison Square Garden (31 October 1932)

Commentaries on the Constitution of the United States, Volume 3, Joseph Story, Hilliard, Grey (1833), § 1784, p. 661

Dennis v. United States, 341 U.S. 494 (1951)

"On the Principles of Political Morality that Should Guide the National Convention in the Domestic Administration of the Republic" (5 February 1784/18 Ploviôse Year 2)
"On the Principles of Political Morality"

Source: No Treason (1867–1870), No. VI: The Constitution of No Authority, p. 12–13
Context: It is true that the theory of our Constitution is, that all taxes are paid voluntarily; that our government is a mutual insurance company, voluntarily entered into by the people with each other; that each man makes a free and purely voluntary contract with all others who are parties to the Constitution, to pay so much money for so much protection, the same as he does with any other insurance company; and that he is just as free not to be protected, and not to pay any tax, as he is to pay a tax, and be protected.But this theory of our government is wholly different from the practical fact. The fact is that the government, like a highwayman, says to a man: Your money, or your life. And many, if not most, taxes are paid under the compulsion of that threat.The government does not, indeed, waylay a man in a lonely place, spring upon him from the road side, and, holding a pistol to his head, proceed to rifle his pockets. But the robbery is none the less a robbery on that account; and it is far more dastardly and shameful.The highwayman takes solely upon himself the responsibility, danger, and crime of his own act. He does not pretend that he has any rightful claim to your money, or that he intends to use it for your own benefit. He does not pretend to be anything but a robber. He has not acquired impudence enough to profess to be merely a "protector," and that he takes men's money against their will, merely to enable him to "protect" those infatuated travellers, who feel perfectly able to protect themselves, or do not appreciate his peculiar system of protection. He is too sensible a man to make such professions as these. Furthermore, having taken your money, he leaves you, as you wish him to do. He does not persist in following you on the road, against your will; assuming to be your rightful "sovereign," on account of the "protection" he affords you. He does not keep "protecting" you, by commanding you to bow down and serve him; by requiring you to do this, and forbidding you to do that; by robbing you of more money as often as he finds it for his interest or pleasure to do so; and by branding you as a rebel, a traitor, and an enemy to your country, and shooting you down without mercy, if you dispute his authority, or resist his demands. He is too much of a gentleman to be guilty of such impostures, and insults, and villainies as these. In short, he does not, in addition to robbing you, attempt to make you either his dupe or his slave.The proceedings of those robbers and murderers, who call themselves "the government," are directly the opposite of these of the single highwayman.In the first place, they do not, like him, make themselves individually known; or, consequently, take upon themselves personally the responsibility of their acts. On the contrary, they secretly (by secret ballot) designate some one of their number to commit the robbery in their behalf, while they keep themselves practically concealed.