The former president's intervention — volunteered during a campaign appearance on her behalf in South Carolina — raised the second, and largely unspoken, issue identified by my friend from the Clinton administration: the two-headed campaign and the prospect of a dual presidency.

In his view, which I share, this is a prospect that will test the tolerance of the American people far more severely than the possibility of the first female president — or, for that matter, the first black president.

As my friend says, "there is nothing in American constitutional or political theory to account for the role of a former president, still energetic and active and full of ideas, occupying the White House with the current president."