In the President's Oath of Office, the Constitution used the normal placement for an adverb – “I . . . do SOLEMNLY swear” and “I will FAITHFULLY execute.”

Wilson Follett in Modern American Usage (the 1960s version edited by Barzun, not the book of the same name by Garner) sets out what has been the standard practice in English for centuries:

"With a compound verb--that is, one made with an auxiliary and a main verb--the adverb comes between auxiliary and main verb . . . . ”

Because of the bogus rule against split verbs, many modern writers move the adverb from its idiomatic place in the midst of a multiple word verb to earlier or later in the sentence.

That is what Chief Justice Roberts unconsciously did when he moved the word FAITHFULLY to later in President Obama’s oath of office.

OBAMA: I, Barack Hussein Obama, do solemnly swear...

ROBERTS: ... that I will execute the office of president to the United States faithfully...

OBAMA: ... that I will execute...

ROBERTS: ... faithfully the office of president of the United States...

OBAMA: ... the office of president of the United States faithfully...

I discussed split verbs in Fear of Writing, 78 Cal. L. Rev. 1677 (1990), my playful review of the sixth edition of the Texas Law Review Manual of Style:

Unquestionably, the most dangerous advice in the old fifth edition of the Texas Manual was its disapproval of split verbs: “Avoid splitting verb phrases with adverbs . . . . ” In other words, don't place an adverb between the parts of a compound verb. Yet Fowler and Follett (both praised in the Foreword to the Texas Manual) argued that the normal place for an adverb is in the midst of a multiple word verb. Thus the fifth edition of the Texas Manual seemed to have gotten the rule backwards. It prohibited what the experts recommend.

This nonsensical rule against split verbs has caused entire volumes of law reviews to be filled with page after page in which adverbs have been squeezed out of their normal place. Most law professors who have dealt with law reviews recently seem either to have had disputes about the placement of adverbs or, worse, to have adopted the Texas approach, the approach of people who write as if English were a second language. It's frightening to think that the ability of a generation of law professors to recognize their native language has been damaged by one silly book. Before picking up the Texas Manual in 1987, I had noticed that the ability of the law reviews to place adverbs correctly had deteriorated, but I hadn't known the reason.

What was particularly ridiculous about the Texas Manual's rule was that the Manual itself repeatedly split verbs in violation of its own rule, a fact that somehow eluded law review editors policing my prose. The only discursive prose in the entire Manual, a four-paragraph Foreword by Charles Alan Wright, contained six split verbs, for example, “their thought can best be expressed.” The Foreword isn't the only place where the fifth edition violated its own rule. Split verbs were common in its text. I found fifteen violations in just four pages, for example, “what has already been said.” The new sixth edition of the Texas Manual has greatly softened its rule against split verbs. It now states:

Splitting verb phrases with adverbs is permissible if the adverb modifies the verb and not some other part of the sentence.

Note that the Texas Manual doesn't say that split verbs are normal or preferable, language it uses to recommend other constructions. Rather, it says that split verbs are permissible. I get the impression that the authors are consciously lowering their standards by permitting but not recommending split verbs. It would have been better if they had admitted their mistake, published an errata sheet for the fifth edition, and begged the academy to forgive them. But the new rule leaves the status of the old rule in doubt. Are split verbs still suspicious constructions in Texas? I think so. The change from the strict old rule is substantial, but given the fifth edition's culpability for the old rule, some effort should have been made to clarify the Texas Manual's current position. Are split verbs preferable or just permissible?

A much better approach would have been to explain the normal placement of adverbs, as Fowler and Follett do. Follett offers a clear statement of the usual practice for the placement of adverbs. His third rule is: “With a compound verb--that is, one made with an auxiliary and a main verb--the adverb comes between auxiliary and main verb . . . . ” Follett goes on to lament the loss of “instinct about the rhythms of the mother tongue.”

You can perhaps begin to see the superiority of the other book under review here, Webster's Dictionary of English Usage, from Webster's discussion of the folklore of the split verb:

Copperud 1970, 1980 talks about an erroneous idea widespread among newspaper journalists that adverbs should not separate auxiliaries from their main verbs (as in “you can easily see” or “they must be heartily congratulated”). This bugaboo, commentators agree, seems to have sprung from fear of the dread split infinitive. Copperud cites five commentators on the subject, all of whom see no harm in placing an adverb between the parts of a verb, and one of whom (Fowler 1965) prescribes such placement. Fowler (under position of adverbs) has a long and detailed discussion, complete with numerous examples in which the adverb has been improperly (to his mind) shifted so as to avoid the split. Since dividing the auxiliary from the verb with an adverb has been approved at least since Lindley Murray 1795, it would seem that Fowler is justified in calling the avoidance a superstition. [FN48]

There it is. The fear of split verbs is a superstition borrowed from some misinformed newspaper journalists. Respected commentators since 1795 are unanimous in finding it proper.

My reference to Miss Thistlebottom is, as I explained in 1990, an image coined by Theodore Bernstein:

The phenomenon that gives rise to such nonsense as the Texas Manual has been well understood by grammarians. H. W. Fowler was content to call such views “fetishes” or “superstitions.” Theodore Bernstein gave them their most colorful term, Miss Thistlebottom's hobgoblins. For Bernstein, the disapproving schoolmarm, Miss Thistlebottom, represented a composite of the type of person who cared very much about good usage, but didn't know it when she saw it. She had a list of supposed infelicities (probably called “pet peeves”) that had been inherited mostly from an oral tradition.