Grouchy old Sen. John "Walnuts!" McCain is always mad, and usually there is one thing in particular that he is mad at at a time. Like for a while it was Ultimate Fighting, for some reason. No one knew why, but he devoted literally all of his time as a senator to eradicating it, until a new thing made him mad and he just completely and totally forgot about UFC. Then for a long time the thing he was mad at was "George W. Bush," and that's when everyone grew to love him, but then he moved on from that, too, and he was mad at Iran for a little while, but mostly it's just been Barack Obama, for the last few years, who really gets his goat. Now he's narrowed his focus further, and the one thing in this world that he hates most is the prospect of Ambassador to the U.N. Susan Rice becoming secretary of state.
This is why John McCain, who has been on the Senate Armed Forces Committee since his first term in the Senate in 1987 (he's currently its ranking Republican), will seek a seat on the other major Senate foreign policy committee next year: the Senate Foreign Relations Committee. The Armed Services Committee oversees the Pentagon, you see, but the Foreign Relations Committee oversees the State Department, and that's where McCain knows Rice is headed. (Unless he successfully blocks her nomination, with his obsequious little weasel of a sidekick Lindsey Graham, that is.) McCain will be much better positioned to constantly badger would-be Secretary Rice, and accuse her of dishonesty and incompetence and probably treason, once he is given his new seat.
John McCain is furious with Rice for her appearances on the Sunday shows the weekend after the attack on Benghazi, where she 1) delivered CIA-derived administration talking points suggesting that the attack happened during, or as an outgrowth of, a protest demonstration outside the consulate, something everyone believed to be true at the time, though it later (later!) turned out to be false, and 2) didn't say "terrorism" or "terror" enough. Because of those dumb Sunday show appearances, McCain has devoted months to attempting to destroy her chances of being named secretary of state. It's very petty and silly and rather blatantly hypocritical, which is to say it is a John McCain crusade.
He has also repeatedly distorted and mischaracterized her remarks and in focusing solely on her has largely succeeded in distracting entirely from every other lingering question related to the Benghazi attack and whatever intelligence failures and operational missteps led to it.
Should he seek a chair on the foreign affairs committee, he'll likely get it. McCain is considered something of a foreign policy elder statesman in Washington, because he is bellicose, hawkish and deeply irresponsible, and he possesses a childish binary "good versus evil" conception of all foreign conflicts that longtime Washingtonians for some reason refer to as "toughness." He would be a fine addition to the committee charged with overseeing our foreign aid and diplomatic missions. After all, it's not like he has a history of very vocally championing certain overseas causes after being lobbied to do so by former aides currently cashing "consulting" checks from foreign governments.