Blackmail, Extortion and a defect in our Language
Extortion is illegally seeking something from someone in exchange for not doing something injurious to them.
Kidnapping for ransom is classic extortion. "Pay us $50,000 or you will never see little Timmy again."
Or a protection racket... "Pay us $200/week or something bad will happen to your restaurant."
"Pay your credit card bill on time or we will charge you an absurdly large fee," is legal, and thus not extortion, but it has the smell of extortion because the fees are unreasonable to any mind that doesn't happen to be a credit card issuer or someone in Congress.
On sub-set of extortion is Blackmail. Blackmail is the extortion of money by a threat of disclosure of information. The classic form is a letter (hence the name) saying, "If you do not want your wife to know about you and your secretary send $500 to this address."
I have a pet-peeve about the way we use "blackmail" to refer to all extortion. When Congress demands spending cuts in exchange for raising the debt ceiling it is (morally) extortion, but when I see someone saying Congress is "blackmailing" Obama it means, to me, that Congress has some embarrassing pictures of Obama at a 1990s Harvard Law Review Christmas party and will sell them to the National Enquirer is Obama doesn't agree to spending cuts.
But, "Congress is extorting Obama," doesn't work because the verb extort is usually used to refer to what is being extorted. You extort money, not a person. (And blackmail wasn't a verb either... but it has come to be a verb.)
So what is the correct verb?