With his victory in North Carolina on Tuesday, Barack Obama took a giant step toward the Democratic presidential nomination. The irony is that he is doing this just when Hillary Clinton has finally exposed his potential weaknesses as a general election candidate.

The Illinois Senator can certainly breathe easier having dodged a loss in North Carolina, where he once held a big lead. As usual, he swept the under-30 crowd as well as the educated, upscale liberals in the central part of the Tar Heel State. He also seems to have...