An example is in Federalist No. 10. Madison is trying to design a political environment resilient to the corrupt effects of factions:

No man is allowed to be a judge in his own cause; because his interest would certainly bias his judgment, and, not improbably, corrupt his integrity. With equal, nay with greater reason, a body of men are unfit to be both judges and parties at the same time; yet what are many of the most important acts of legislation, but so many judicial determinations, not indeed concerning the rights of single persons, but concerning the rights of large bodies of citizens? and what are the different classes of Legislators, but advocates and parties to the causes which they determine? Is a law proposed concerning private debts? It is a question to which the creditors are parties on one side and the debtors on the other. Justice ought to hold the balance between them. Yet the parties are, and must be, themselves the judges; and the most numerous party, or, in other words, the most powerful faction, must be expected to prevail. Shall domestic manufactures be encouraged, and in what degree, by restrictions on foreign manufactures? are questions which would be differently decided by the landed and the manufacturing classes; and probably by neither, with a sole regard to justice and the public good. The apportionment of taxes on the various descriptions of property is an act which seems to require the most exact impartiality; yet there is, perhaps, no legislative act in which greater opportunity and temptation are given to a predominant party, to trample on the rules of justice. Every shilling, with which they overburden the inferior number, is a shilling saved to their own pockets.

It is in vain to say, that enlightened statesmen will be able to adjust these clashing interests, and render them all subservient to the public good. Enlightened statesmen will not always be at the helm: Nor, in many cases, can such an adjustment be made at all, without taking into view indirect and remote considerations, which will rarely prevail over the immediate interest which one party may find in disregarding the rights of another, or the good of the whole.

His concrete solutions are to choose representative democracy over direct democracy, and to have large republic rather than a small republic.

A more recent example would be last year's ban on members of Congress trading stocks based on the inside information they have as lawmakers. I think Milton Friedman's point is that one should direct efforts toward supporting policies like that, rather than trying to elect politicians who are too ethical to insider-trade.