In the recent past, American English has resuscitated what had fallen into disuse in the grammatical range of the verb advocate, namely its intransitivity, with the concomitant government of the postposition for. What the Century Dictionary and Cyclopedia marked as rare in 1906 has come back as the dominant syntactic profile of this verb, as in:

“It is an organization that has made a decision to cast aside its journalistic integrity and to advocate for the defeat of one candidate … and advocate for the election of another candidate,” he [a spokesman for the McCain campaign] said. (“McCain Aide Blows Gasket, Rips New York Times,” Jimmy Orr, “The Vote Blog,” The Christian Science Monitor, 9/22/08)

It is in fact this syntax that has all but displaced the traditional transitive government of the verb.

The Oxford English Dictionary entry shows that what may seem to be a contemporary innovation is actually a resuscitated archaism, witness the following attestations:

1. intr. To act as advocate, to plead for. arch.
1641 MILTON Animadv. §1 (1847) 58/2 It had been advocated and moved for by some honourable and learned gentlemen of the house. 1659 FULLER App. Inj. Innoc. (1840) 339 I wonder that the Animadvertor will advocate for their actions, so detrimental to the church. 1661 HEYLIN Ref. I. ii. 37, I will not take upon me to Advocate for the present distempers and confusions of this wretched Church. 1872 F. HALL False Philol. 75, I am not going to advocate for this sense of actual [i.e. as = present].

As a curious sidebar to the story, the transitive meaning, which we take for granted and as needing no exemplification, was one that Benjamin Franklin found worthy of “reprobation,” as in the following OED attestation given under 3. trans.To plead or raise one’s voice in favour of; to defend or recommend publicly:

1789 FRANKLIN Lett. to N. Webster 26 Dec. Wks. 1840 X. 414 During my late  absence in France, I find that several new words have been introduced into our parliamentary language. For example I find a verb..from the substantive advocate; the gentleman who advocates or has advocated that motion..If you should happen to be of my opinion with respect to these innovations you will use your authority in reprobating them.

How to explain this resurrection of an archaic meaning? We need to examine what transitivity implies that intransitivity does not.

When a verb governs the direct object, the action of the verb is transferred directly onto the object without any mediation. Not so an intransitive verb, since there is no direct object. As a matter of fact, as in the case of advocate for, if a postposition mediates between the verb and its object, there is necessarily an ATTENUATION OF THE FORCE of the verb, such that here the act of advocating is necessarily less forceful than it is when the verb is used with its traditional transitive syntax.

Thus the contemporary intransitive syntax of advocate with the postposition for takes its place among constructions like lobby for, plump for, speak for, even search for, etc., where the activity denoted becomes generalized, hence attenuated. “Advocating for world peace” is not the same as “advocating world peace:” the first denotes one of those vague sets of actions that peace-loving people (“peaceniks” et al.) engage in; the second involves a definite commitment.

Speaking of commitment, it is interesting to note the converse directionality of the change in modern (mostly American) English that has resulted in all modern dictionaries dropping the traditional obligatory government of commit that requires the reflexive form self (commit oneself/herself/themselves, etc.). And just as in the case of advocate for, where the intransitivity connotes a weakening of the verb’s force, here the analysis is exactly parallel: the dropping of an obligatory reflexive after commit implies that the original force of the verb meaning ‘pledge/bind oneself’ has been denatured so as to mean an action that is less than binding. The new syntax in both instances is an icon of the new semantics.