After half-heartedly trying to pick a fight, I've concluded we're in pretty close agreement here. (Which means we're probably wrong.) We agree especially on Caro's failed attempt to make compassion an alternative motive for Johnson, a motive that competes with his ambition—that wars with his ambition as the good side of his nature warring with the bad, like the LOVE and HATE on Robert Mitchum's knuckles in Night of the Hunter. Yet I do quibble with your claim that the triumph of LBJ's ambition makes his compassion "wholly phony." It's not necessarily phony; it's just rendered not very relevant.
A deeper problem with the compassion theme, I think, is that in general it's a weak explanatory concept. Don't all politicians think they're acting compassionately? Our incumbent's "compassionate conservatism" is a vapid slogan because it implies that most conservatives are self-consciously cruel or indifferent to suffering. But the slogan is also effective because liberals still own the word "compassion"—which remains, in effect, synonymous with government intervention to solve social problems—and so it makes Bush sound more original than he really is. (The slogan led some credulous reporters to think Bush was actually a moderate, but it turned out he meant only that he thinks cutting taxes for the rich, privatizing Social Security, etc., are compassionate policies. As a conservative should.)
This digression on Bush isn't an attempt to generate disagreement between us. I brought it up because I think Caro's equation of compassion with racial equality, and with postwar liberalism in general, reflects a certain worldview that helps explain his love-hate relationship with LBJ.
Although he's never pedantic, it's not hard to know where Caro's own politics lie. He's refreshingly candid about it. Not only does he make fun of Kenneth Wherry, as you note; he's openly disdainful of other Midwestern conservatives like John Bricker and Homer Capehart, in ways that made me laugh out loud. At one point he even referred to Capehart off-handedly as "the Indiana Neanderthal" with no further explanation. I've never seen a picture of Capehart, and all I know about his appearance is that he was fat. Maybe that's why I imagine him as looking like Homer Simpson.
On the civil rights fight, Caro's sympathies clearly lie with Paul Douglas, Herbert Lehman, and the senators Johnson considered "crazies." No problem there. Liberals can't gloat over much these days, but being on the right side of the civil rights struggle remains a point of pride. These liberals hated Johnson. When Joe Rauh was elected chairman of Americans for Democratic Action, he devoted his acceptance speech, Caro says, to trashing the majority leader. (Not every liberal hated LBJ: There was Humphrey, whose capacity for obsequiousness, as you indicated, rivaled Johnson's own; and Helen Gahagan Douglas, who Caro intimates had an affair with Johnson—a grotesque thought that boggles the imagination, given the beauty-beast contrast between them. Maybe she was seduced by his power?)
Caro's feelings toward LBJ echo these liberals'. There is awe. But there's also resentment at his capricious and self-serving use of power; and anger at his readiness to humiliate others—as he needlessly did to Paul Douglas once by calling for a roll-call vote (as opposed to a routine voice vote) on a doomed motion of Douglas', forcing Douglas' fellow liberals to vote against their ally or face LBJ's retribution. After being defeated 76-6, Douglas hurried to his office, where he closed the door and wept. You asked me in your first entry if Caro likes Johnson. He wouldn't spend three decades of his life writing about the guy if he didn't like him in important ways. But anyone who would lay out the story of Douglas' humiliation in the exquisite detail that he does plainly hates Johnson as well.
For all my own affinities with these 1950s liberals, I've long faulted them for not appreciating the need to fight hard, to seize opportunities, to exercise power vigorously. Postwar liberalism has contained a strain embodied by Adlai Stevenson (as I wrote about once in Slate): a squeamishness about the use of power, a belief that it's better to be pure than to win. This trait was on display in the 2000 election aftermath, when Bush played no-holds-barred politics, while Gore took the moral high ground, forsook opportunities to win, and got rolled. Say what you will about Johnson, but—like Franklin Roosevelt and John F. Kennedy—he enjoyed using power and played to win. And although he used it for selfish ends, those selfish ends did happen to do a lot of good. Caro knows this, but I don't think he's accepted it.
This is also why I don't find Johnson tedious. The scenes you cite of him wielding his power over friends and enemies alike, of using physical and psychology maneuvers in the service of his ends, suggests to me a complex, resourceful character. Often annoying, often ugly, often appalling—but more than "colorful." As you say, power does not always "reveal" in the open-sesame way that Caro proposes. But in the person of Lyndon Johnson, and as described in the engulfing prose of Robert Caro, it is decidedly compelling and fascinating to watch.
It's been a pleasure exchanging thoughts, Chris. I move to adjourn.