AS A MATTER of political strategy, Sen. John F. Kerry's choice of running mate is a smart move. North Carolina Sen. John Edwards, to put it bluntly, is strong in areas where Mr. Kerry is weakest. In that sense, it's to Mr. Kerry's credit -- as well as a measure of his determination to beat President Bush -- that he was willing to enlist a man who, to some extent, showed him up during the primaries even though Mr. Edwards lost resoundingly. While Mr. Kerry can seem stiff and aloof, Mr. Edwards, as he proved during the primaries, is a natural campaigner who deploys the skills he honed as a trial lawyer to connect with voters.

Mr. Kerry is the product of a privileged upbringing whose patrician demeanor is no asset on the campaign trail. By contrast, Mr. Edwards, as he never tired of telling audiences during the primaries, is the son of a millworker, the first in his family to go to college. With Republicans portraying Mr. Kerry as a Massachusetts elitist, the addition of Mr. Edwards to the ticket strengthens the Democrats' appeal not only in his native South but in rural communities nationwide (his campaign song was John Mellencamp's "Small Town"). And as Mr. Kerry has struggled to articulate a coherent campaign theme, Mr. Edwards managed to encapsulate his in the simple trope of the "two Americas." In many ways, the two men are like jigsaw puzzle pieces that fit snugly together: Melded, they would make a nominee superior to either man standing alone.

Yet lacking the ability to perform such political alchemy, we must also assess Mr. Edwards on his own and by the grave question that has to be asked of any vice presidential nominee: Is he ready to assume the presidency? This is a question that, since Sept. 11, 2001, has become both more thinkable and more important -- and it's one that, when it comes to Mr. Edwards, we can't yet answer with a resounding affirmative. Mr. Edwards's often impressive performance during the primaries demonstrates that he has the capacity to grow into the job. Yet his experience in public life is confined to the single Senate term that is now ending; his few years on the intelligence committee represent scant training in foreign policy and military affairs, far less than would be optimal for a potential president in this dangerous time.

Mr. Edwards improved as a candidate during the primaries, but he also demonstrated, especially toward the end, an unfortunate tendency to cater to popular but irresponsible positions. Mr. Edwards was powerful, even moving, when he spoke about how the country was divided into two Americas, with one tax system, one school system, one health care system for the rich, and an inferior version for everyone else. Yet his lurch toward protectionism on trade was disappointing for a candidate who we thought knew better. When the two were the last plausible candidates standing, Mr. Edwards sought to make trade the defining difference between them. "Senator Kerry and I have very different positions on the issue of trade," Mr. Edwards said. Mr. Kerry responded to all this in a way that diminished his candidacy, railing against "Benedict Arnold CEOs" who outsourced jobs abroad while trying to obscure his commendable record on free trade.

Now that the two are a team, we hope their interaction will take on a different character -- not to reinforce each other's worst political instincts but to bring out their best political selves. If Mr. Edwards can help humanize and energize Mr. Kerry as a campaigner, if his notion of uniting the "two Americas" can help Mr. Kerry sharpen his campaign theme without veering into class warfare, then Mr. Kerry will have made a wise choice indeed.