Clifford Orwin is professor of Political Science and senior fellow of Massey College at the University of Toronto.

Although it has been 15 years since the attacks of Sept. 11, their harsh afterglow still pervades politics in the United States. Shock, anger, an understandable thirst for retribution, a longing for a return to normalcy coupled with dread of a persisting menace: All play their part in the current presidential election.

Since that black day, Americans have spent an estimated $1-trillion (U.S.) on enhanced security. Inevitably, much of that has been wasted, but, in fact, the United States is safer now. It remains vulnerable, however, to a dirty bomb or biopathogen, among other dire threats.

In the meantime, the predominance of terror attacks has shifted, from the al-Qaeda model of spectacular catastrophic assaults, to the Islamic State model of a proliferation of smaller attacks on soft undefended targets. These attacks can neither be deterred nor reliably detected beforehand. Their persistence defines the new normal in North American terrorism. The elimination of Osama bin Laden, while gratifying as retribution, proved largely irrelevant otherwise. Meanwhile, President Barack Obama (and the world) have paid dearly for his premature dismissal of the Islamic State. Since its explosive success and rapid metastasis, its terrorists enjoy ample havens, while the Internet incites the aggrieved and alienated in every drab U.S. basement.

Enter Hillary Clinton and Donald Trump, each running in their different ways against the phantom of Sept. 11. Both were middle-aged at the time, with established public personae. Response to the attacks, however, has been central to the careers of both.

Most importantly, Ms. Clinton voted for the Patriot Act, for trying suspected terrorists abroad and for the Senate resolution enabling the Iraq war. That resolution was thinkable only in the context of the attacks. Her vote did not express her supposed "hawkishness." As a senator from New York she naturally dreaded the weapons of mass destruction that all intelligence agencies assumed Saddam Hussein possessed.

Yet, if the aftermath of Sept. 11 thus spawned the lingering cloud over Ms. Clinton's record in the Senate, it also yielded her only solid success. That was her championing of federal compensation for survivors of victims and for first responders sickened by the fumes. While promoting your constituents' interests is the first priority of any Senator, the link with Sept. 11 lifted this case far above the humdrum.

For all his fecklessness, however, Mr. Trump has proved at least as savvy as Ms. Clinton at plucking the chords of Sept. 11. In the immediate aftermath of the attacks, his role was not a prominent one. Now, however, the Islamic State having upstaged al-Qaeda as the main threat to domestic tranquillity, he blames its rise on Mr. Obama and Ms. Clinton (which is more plausible than they will admit). His promise to defeat it in 30 days, while empty, is resolute and implicitly vindictive. (You don't defeat terrorists so quickly except by blasting them to smithereens.)

Yet, typically for Mr. Trump, this promise also caters to the longing for normalcy and implicitly for isolationism. After 30 days of Mr. Trump, the Islamic State will have vanished just like America's other problems. Some enemies you build walls against; others you just cream.

There's another legacy of Sept. 11 to which Mr. Trump appeals. The attacks sowed mistrust not only of Muslims but of the U.S. government. Where was it when Americans needed it? Must there not have been a conspiracy? The subsequent revelation of missed opportunities to foil the plot and the failure to hold anyone accountable for them only stoked this mistrust. So did the Iraq war. Mr. Trump has long blamed George W. Bush for both, and Ms. Clinton for her complicity in the war. It's the saddest irony of Sept. 11, which briefly united Americans (and which unity Mr. Bush briefly succeeded in expressing) that its consequences so contributed to their ever-deepening divisions.

Mr. Trump, the relentless exploiter of those divisions, who runs against both parties while playing both ends against the middle, thus may succeed in out-9/11-ing Ms. Clinton.