Great rhetoric works best when it is not innovative, but summons us to the best that is already latent in our public memory.
One might also add to the examples Ralph gives Lincoln's Second Inaugural -- which clearly quotes Mt. 18:7 and Ps. 19:9 and alludes to Gen. 3:19 (the particular form the allusion takes shows that Lincoln is actually alluding to a previous speech in which he alluded to Gen. 3:19) and Mt. 7:1. The 'charity towards all' part is certainly derived from elsewhere, although it's sufficiently widespread at the time that a particular source might not be discoverable (but its use here is very probably influenced by 1 Peter 4:8-9; note the point about charity covering a multitude of sins). The mention of the soldier's widow and orphan taps into a long tradition of prophetic and ministerial discourse about caring for the widows and the fatherless (cf. Zech 7:10, James 1:27) that would have been recognized. Nonpartisan, to whom Ralph is responding, makes clear in the comments that Scriptural quotations are being set aside for the purpose of that argument, which focuses on quotation from the speech of other politicians; that, no doubt, is why Kennedy's "Ask Not" speech made it onto the list, since Kennedy very explicitly quotes Isaiah 58:6 and Romans 12:12. So Ralph's point needn't be seen as problematic for Nonpartisan.
But the point is important, I think, and there is an additional speech explicitly mentioned by Nonpartisan that fits it very well. Washington's Farewell Address may have no direct quotes, but it has some very clear allusions. The most obvious is the claim that virtue or morality is a necessary spring of popular government; the formulation of the maxim in terms of the 'springs of government is due to Montesquieu (The Spirit of the Laws, Book III), and since Montesuieu is widely read, it is a common claim. We find Robespierre, for instance, using very similar words just two years before (although he confines the maxim to times of peace; in times of revolution, he says, you need virtue and terror, which is a somewhat different twist). It's not a direct quotation of Montesequieu; but it's very, very close to being one.
It makes me wonder, though, if the problem is not originality or quotation of people who are effectively equals or even inferiors, as Nonpartisan suggests, but the fact that we have no political theorists worth quoting. You can build a government on Montesquieu; in the eighteenth century you could hardly do better. Even now, the primary difficulty with quoting him is just that he's over two hundred years old and no longer read by everyone interested in politics. Due to the fact that it has ongoing relevance due to reinterpretation, preaching, and the like, you can do much the same with the Bible; you have to select with good sense and good art, but you can tap into some powerful political thought by quoting the Bible. (MLK, Jr. is an excellent example of this; his task in this was made easier in part by the Social Gospel and political theological work with which he was very familiar and in part by the widespread use of the pulpit as a point from which society and sometimes government might be thoroughly critiqued.) But, honestly, what do we twenty-first century Americans have to build on? The Bible still has some residual force, perhaps; but setting that aside (and it is clear that, for a number of reasons, it would only take you so far these days, anyway) we really don't have any discourse you can build a government on. That takes both philosophy and poetry, and our talk about government has had precious little of either for a long time. And that's what's missing, I think.
