I've been meaning to comment on this issue for some time. Crucial tasks related to a book I've written have been among the many reasons for delay (see http://faceofother.blogspot.com for details). Now that McCain is about to pick his running mate, I thought I'd better comment before my comments are totally out of date.
What if McCain picks Romney? Like Romney, I'm a Latter-day Saint ("Mormon"). Would that lead me to reconsider my support for Obama? Not in the slightest.
I know Mitt Romney. Though he served a mission well before I did, we both served in the France Paris Mission. Like me, he was an English major at BYU. When I went to graduate school in Massachusetts (late 1970s, early 1980s), he and I were both members of the Boston Massachusetts Stake--that's a church unit that includes several congregations. He was in the Stake Presidency, and I chatted with him at least twice during temple recommend interviews. I remember vividly some of what we talked about. Since I left Massachusetts, I talked to him at least once, when he visited BYU to give an "honored alumnus" talk. Later, in 1997, while I was doing research in Massachusetts, I got to know a couple of his sons. So whether he remembers me or not (and I think he probably does, at least vaguely), I feel like I know him.
I was impressed by his moderate political stance before, during, and after his time as governor of Massachusetts and by his skill in rescuing and running the 2002 Winter Olympics. But over the past year or so, I've gotten a lot less impressed. He has shifted far to the right, and I'm convinced he's done so not just because he was converted to different opinions on social issues (I think that conversion was probably genuine) but because he wanted to get the Republican nomination. In debates he seemed to try to position himself as far to the right as he could, claiming he would double the size of the Guantanamo Bay prison camp, trying to sound as grim and mean-spirited as he could in talking about foreign policy, and having no problem with treating imprisoned suspects in ways resembling torture. (By the way, if we apply the standards the Bush administration favors for such treatment, most of what the North Vietnamese did to John McCain would not count as torture. I think it should.)
In general, I have had a hard time feeling Mitt Romney is really speaking from the heart. Rather, he sounds like a politician who says and does what he thinks will work to advance his career. I'm afraid he strikes me these days as opportunistic and even shallow.
But I still like him personally. He has a wonderful family. He's a good man, with flaws, of course. And he's very bright--and an extraordinarily capable businessman. But if he were to run as a vice presidential candidate, that would emphatically not make me more likely to vote for McCain. If anything, it would make that prospect even less attractive.
On the other hand, he'd be better than some of the other people being considered: Giuliani (I'm not impressed by his character, his personal life, his positions on some issues, and his tough guy stance, something I find silly in some ways and certainly not the best way to deal with the world's problems); Tim Pawlenty (a bit thin); Sarah Palin (super thin--meaning inexperienced); Joe Lieberman (mean-spirited as well as too liberal on social issues).
At least Romney is a capable manager with a good deal of experience in both business and government. He's well spoken, though sometimes it's hard to tell how deeply he believes what he's saying.
In any case, none of those being considered by McCain seems to me a match for Joe Biden. So in short, the vice presidential dimension of this race makes me feel even more comfortable with Obama.
P.S.: While writing this, I've learned that Sarah Palin will be McCain's running mate. It's great that we'll have a woman on one of the tickets. But she's awfully inexperienced, especially on the national or international stage. Obama has lots more experience than she does, especially at those non-local levels. Especially given McCain's age, his running mate needs to be ready to be president. Palin is definitely not. But I'm sure she's a likable, good person.
P.P.S.: [Added much later, after the Republican convention:] I know Margaret wanted Romney to be the vice presidential candidate, but I'm glad he's not--for several reasons. I've been very disappointed by his continued attempt to demonize and dehumanize "our enemies." A recent example: At the Republican convention he called the current Supreme Court too liberal, because they decided that constitutional protections should be accorded to the prisoners at Guantanamo Bay. It occurred to me that instead of reading L. Ron Hubbard (he called one of his books his favorite) he might try the classic play A Man for All Seasons, with its reminder that denying legal protection to our enemies not only is wrong but endangers us as well. (I'll provide the quotation in a comment.) The worst horrors of human history have happened when people have convinced themselves that their enemies don't deserve to be treated as fully human. And free societies have lost their freedom when fear has led them to sacrifice legal protections for the sake of safety. Ben Franklin had a good one about that: "They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety."