THE COMBINATION of President Obama’s last months in office and the presidential campaign has unleashed a flurry of debate about nuclear weapons. Republican nominee Donald Trump has suggested he might withdraw the U.S. nuclear umbrella from allies such as Japan and South Korea, and his combative style has raised the specter of a hothead with his finger on the button. Meanwhile, Mr. Obama is considering whether to make a “no first use” declaration about nuclear weapons, and may seek renewed support for a Comprehensive Test Ban Treaty at the United Nations.

In one way or another, all of these touch on important aspects of nuclear weapons policy. It is obvious that Mr. Trump is being downright reckless, and Mr. Obama may be trying to polish a legacy that never quite fulfilled his 2009 Prague speech proposing a new era of nuclear disarmament . But the remaining weeks of the campaign would be better spent with serious debate about the real problems facing the new president.

At the top of that list is an expensive modernization of the U.S. strategic nuclear deterrent. Updating and replacing weapons that date back to the Cold War is essential, but the next president will have to make tough choices. For example, the Navy is embarking on an ambitious program to build 12 ballistic-missile submarines to replace the existing 14 Ohio-class “boomers,” the most invulnerable leg of the strategic triad. But the $97 billion price tag for the replacement fleet threatens to soak up Navy funding for other programs such as attack submarines, destroyers, aircraft carriers and amphibious warfare ships. In order to do it all, the Congressional Research Service has estimated Navy shipbuilding budgets would have to be boosted by a third over historic levels. Can the United States afford to have it all? This question hangs over the Air Force, too, which is working on a new strategic penetrating bomber and wants a new long-range cruise missile. The missions of these two weapons systems may overlap: Is the cruise missile necessary?

At the same time, Russian President Vladimir Putin and his confrontational approach have thrown into doubt earlier cooperation on arms control and nuclear security. Russia’s violation of the Intermediate-Range Nuclear Forces treaty is unresolved, and Moscow appears to be designing asymmetric weapons such as a nuclear-capable underwater drone, as well as building new missiles and submarines. North Korea has a steadily expanding nuclear arsenal and missile program. Nuclear deterrence is still essential and will be for some time. While continuing U.S. modernization and keeping a wary eye on Mr. Putin, a new president should look for specific areas for engagement with Moscow, such as keeping nuclear material out of the hands of terrorists and reducing the dangers associated with both nations’ launch-ready alert postures, largely unchanged since the Cold War.

Mr. Obama’s early vision of a world without nuclear weapons is a long way off. It is time to work on present-day reality: What kind of strategic nuclear weapons do we need, at what cost and to deter what kind of threats? The campaign could use a debate that acknowledges this and grapples with it.