he difference between running for president and getting picked as a vice-presidential nominee is the difference between physics and chemistry.
When George W. Bush and John McCain were dueling banjos, racing around from primary to primary yanking all the levers of publicity and politicking, that was physics, the science of matter in motion.
But when the pair meet Tuesday in Pittsburgh for their long-delayed bury-the-hatchet session, the outcome depends upon chemistry, where the interaction of different substances relies on totally different laws.
It's the difference between wholesale and retail, selling yourself as No. 1 to millions of anonymous voters, on the one hand, and selling yourself as No. 1 to one other politician - or selling that person on being No. 2, behind you. It requires a different set of skills.
McCain has said umpteen times he's not interested in being vice president, never-never-never-cross-my-heart-and-hope-to-die. Bush has said he'll only pick a running mate who likes him. You can toss both those claims in the trashcan.
McCain could be persuaded, I believe, if it were done in the right way. How's that? Putting it to him as a matter of patriotism, honor, duty, all the stuff that's crammed into McCain's speeches, his autobiography, his Victory Lap tour of book signings.
But I am yet to be persuaded that Bush wants McCain, or feels he needs McCain, to win the White House.
In the early going, when Bush was way ahead, and had all the money anyone could conceivably need, and nearly the whole of the Republican establishment was whispering sweet nothings in the Texas governor's ear, his body language toward McCain was confident, matey, jocular.
''John's mah buddy,'' Bush would chirp at every photo op. ''We get along fine.''
But then McCain's numbers started zooming up, and Bush's numbers got all weak in the knees in New Hampshire and then New England, and Bush lost a few, and got rattled. Like his father did in 1988, Bush Jr. went negative, and it worked, like it almost always works. McCain got rolled up, and since the March primaries that ended the war, each man has gone his own way.
McCain has kept his distance artfully, setting up his Straight Talk America political action committee, revisiting the Hanoi Hilton on a lavishly-documented Vietnam War anniversary tour, and promising McCainiacs that he'll never go in the tank on the gut issue of campaign finance reform.
We'll get to know Bush's choice before Al Gore's because the Republican convention comes at the end of July in Philadelphia, and Gore has another two weeks before the Democratic convention in Los Angeles. It is a commonplace assumption within the political crowd that McCain would be the strongest choice Bush could make. But that view is by no means unanimous.
McCain's pluses are that he is popular with moderates and Democrats, the people who can be moved to vote for you in November; that the Arizona flyboy has demigod status with a lot of the media; and that his well-honed stump skills will maximize his potential to, as he so deftly put it in a thousand stump speeches, ''beat Al Gore like a drum.''
So what's the downside?
McCain is a maverick. Though he survived in the military, a heirarchal system at least as strait-jacketed as the vice-presidential role, he was always a rebel, a rock 'n' roll wave-maker, and apple-cart-upsetter.
It remains to be seen if Bush can feel secure and serene with McCain bouncing around the playing field. Presidents like to have their vice presidents shut up for long stretches. Bush Sr. bit his tongue for Reagan for eight long years. And Al Gore's seven long and lean years of apprenticeship under Clinton cemented him in the role of Clinton's understudy.
Bush also has to think long and hard about what happens if a Bush-McCain ticket loses to the Democrats. Does that set McCain up to be a lusty rival for the GOP nomination in 2004? And what does a McCain selection as running mate portend for the other Republicans who'd like to take a crack running against President Gore next time, if it comes to that?
Presidential nominees look at running mates with a whole new set of spectacles. You have to set aside the microscope and pick up the telescope: How far can I go with this person?
The classic cases of nominees taking bitter foes onto the ticket are JFK and Reagan. Kennedy embraced Lyndon Johnson after being persuaded he needed the Southerner to balance the ticket sufficient to win the Electoral College. Reagan had formed a low opinion of Bush Sr. after their hilarious encounter in the Nashua, N.H., high school gym where Bush, with ungentlemanly ill-grace, refused to share the microphone with Bob Dole and three other rivals, triggering the Gipper's famous ''I'm paying for this microphone ...'' outburst.
But in a Detroit hotel room on the eve of the 1980 convention, Reagan was persuaded that only by embracing Bush, the candidate of the Eastern Establishment and the Non-Voodoo School of Economix, could he dump an incumbent president. So there's ample precedent for taking a former enemy on as your co-pilot, if that's what it takes to win.
Bush has alternatives. There's Pennsylvania Governor Tom Ridge, a low-key, pro-choice Roman Catholic, popular in a state Bush needs to win. The electoral college map can never be discarded in this kind of game. The no-better-than-slim-outside-chance that Bush could carry New York state probably dooms the chances of that state's governor, George Pataki.
But all those considerations of past tangles, ideological friction, and electoral map-making pale in comparison to the level of personal security the No. 1 man has in contemplating his choice of No. 2. If Bush really wants McCain, he should make that plain Tuesday in Pittsburgh. It's not hard to write a scenario in which McCain could find it impossible to say no to a summons to duty, honor, country.
We should know Tuesday night if Bush is smart enough, secure enough, and strong enough in his own persona to take that step.
David Nyhan is a Globe columnist.
