In a way, it's a tradition as old as Jefferson's paranoia about Hamilton (or, later, Burr). It never really goes away, and every once in a while it surfaces into the mainstream.** And it's never pretty.
The latest manifestation, the 'Tea Party'/deathers/birthers/whatever, while ugly, is pretty much par for the course. But there are signs that we may be passing into new territory ("new" for our lifetimes, that is --things have gotten this bad, and worse, in what we naively think of as the good old days***).
First, and most recently, there's the "Psalm 108.9" movement, in which faux-Xians pray for the president's death.
This reminds me very much of a saying of Janice's, that "Anytime an American tells you 'this is a Christian nation', it's a sure sign they're about to go all Old Testament on you".
And second, there's the serious suggestion, about two months back I think, that this country desperately needs a military coup to overthrow the government.
As if losing an election were reason enough to call for an act that would permanently damage our constitutional system. As if that system didn't have its own corrective (impeachment). As if we've had a general since Jackson who rose above mediocrity as president. As if you could dismiss democracy itself -- letting people vote to choose their leaders-- as "gambl[ing] the national survival on . . . political whims".
*I must admit the 'secret wife' bit was a new one on me.
**Heinlein got involved in one iteration, a brief anti-Eisenhower eruption calling themselves 'the true heirs of Patrick Henry'.
***For example, Ambrose Bierce was deeply embarrassed by McKinley's assassination, since he'd been calling for McKinley's death in his newspaper column.