Suppose that, as October 1 rolls around, Senator Obama is leading Senator McCain in the polls by 10 percentage points. The situation in Iraq is much as it has been — moving in the right direction but dangerous nonetheless. American generals say that Iran has been funding attacks on American troops on the Iraq field of battle. Iranian gunboats are regularly harassing American navy vessels in the Persian Gulf, and Iranian-backed terrorists are lobbing mortars and rockets at the Israeli city of Sderot. Iran is enriching uranium to build an atomic bomb.

President Bush will then face a choice. Does he launch an attack on Iran's nuclear sites or regime command and control targets, hoping either to decapitate the Iranian leadership and achieve a regime change there or to set back the Iranian nuclear program by several years, before the Obama administration takes office? Or does he leave the problem to Senator Obama, who has pledged to negotiate directly with President Ahmadinejad?

It's not an easy decision. On one hand, Mr. Bush is a politician who probably thinks that the candidates to succeed him deserve to battle it out without a campaign-changing event by the incumbent. He has also seen, in the battle of Iraq, that military action can sometimes be more complicated than ever initially imagined. He knows that the United States Congress is in the hands of Democratic leaders who have asserted, however wrongly under the Constitution, he lacks the authority to attack Iran.

Yet Mr. Bush also takes seriously his obligation to make a success out of Iraq, and, more importantly, to protect America's security. A nuclear-armed Iran, led by the mullahs who now control the country, would be a serious threat to the national security of America and of American treaty allies in Europe and to our friends in the Middle East. Mr. Bush would have doubts about the fortitude of a President Obama in the face of such a threat, and would certainly be at least tempted to act were he to conclude that he could extend by several years the envelope within which America would be safe from a nuclear-armed Iran. We are not making a prediction that Mr. Bush will move against Iran but fate may yet take things in hand.