Form of Participle Clause Subject: 'his' being or 'him' being

I have come across the following sentence (a bit modified) in a news article: "The opposition leader's bid for power failed to gain traction, and his claim to the presidency is rooted in his being the head of the current legislature." I wonder why the subject of the participle clause "'his' being the head of the current legislature" is "his", but not "him." Maybe, both "his" and "him" could be used in this particular sentence--I have no idea. Then, is there any differences between them in meaning, grammar or any other aspect? In my understanding, a participle clause can have its own subject, as is the case above. Most of the time, the "him" form is used, but also "his" can be found, albeit less frequently. I am particularly interested in differences in meaning between them when both forms are applicable, and in what kinds of rules are applied when determining which form to use. Thanks for your valuable knowledge.

Solution 1:

The opposition leader's bid for power failed to gain traction, and his claim to the presidency is rooted in his / him being the head of the current legislature.

The pronoun subject of gerund-participial clauses functioning as a complement can be either genitive case or accusative case. The only difference is one of style, with the genitive case being characteristic of fairly formal style. Syntactically, the pronoun "his / him" is subject and "being the head of the current legislature is a verb phrase functioning as predicate.

Note that when a gerund-participial clause is in adjunct function, genitive subjects are not permitted at all: the choice is between nominative and accusative:

She sought advice from Ed, he / him being the most experienced of her colleagues.