Constitutional lawyer Edwin Vieira Jr. declared in a column, "America is facing potentially the gravest constitutional crisis in her history." His reasoning (or lack thereof) is based on his perception that Obama has not proven to the satisfaction of every single wingnut in the country that he wasn't born on some outer planetoid in the asteroid belt. He described an Obama presidency as a "national disaster of having an outright usurper purportedly elected as President." Vieira, BTW, is the brother of TV news personality Meredith Vieira and possesses BA, MA, PhD, and JD degrees from Harvard University, proving that a mutational variety of wingnut does exist.
There is good news for Obama, according to Vieira:
If Obama does become a usurper posturing as the President, Congress cannot even impeach him because, not being the actual President, he cannot be "removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors" (see Article II, Section 4).
However, Vieira goes on to argue for options besides impeachment:
In that case, some other public officials would have to arrest him—with physical force, if he would not go along quietly—in order to prevent him from continuing his imposture.
I can just imagine someone like Alan Keyes (who has already filed his own lawsuit) getting up on a table at a nearby beer hall and organizing a putsch; marching down Pennsylvania Avenue with other wingnuts like the mob scene in a Frankenstein movie.