“If George W. Bush wants to make national security the central issue in this campaign,” Democratic presidential candidate Senator John Kerry (D-Mass.) says at campaign events across the country, “I have three words for him I know he understands: Bring it on.”

By boasting that he will stand up to George W. Bush on national security, Kerry has brought Democratic partisans at rallies to their feet and on primary day to the polls. In the next two weeks, Kerry almost certainly will wrap up his quest for the Democratic nomination, and immediately, the debate he seeks with Bush on security will begin.

For the first time in two decades, foreign policy will be a central issue of an American presidential campaign. The attacks of September 11, the ensuing war against terrorism, and especially the invasion and occupation of Iraq have placed this issue context at the top of voters’ concerns by turning national security into a matter of personal security.

National security is no longer an issue of submarines in the Baltic or the number of warheads on a missile. To Americans, it’s now a question of whether it’s safe to board an airplane or even to visit their nation’s capital.

That’s why even a conflict as prominent as the Israeli-Palestinian one won’t move many voters beyond small numbers of Arab- and Jewish-Americans. Instead, the central dynamic of the foreign policy debate this election year will be: whom do you trust to keep America safe?

Ever since 9/11, Bush and the Republicans have enjoyed a huge advantage on national security. A CNN/Gallup/USA Today poll taken at the beginning of this year found that Republicans enjoyed a 51 to 40 percent lead over Democrats on foreign policy and an astounding 60 percent to 30 percent advantage on the war on terrorism—margins unseen since the 1980s.

Driving these numbers is the fact that voters like and trust President Bush to keep them safe. Yet in what could be a precursor to the campaign season, that trust is beginning to erode.

The Kay Report detailing the lack of weapons of mass destruction in Iraq, revelations of Halliburton over-billing the Pentagon, and efforts to fudge economic numbers about everything from when the recession started to the number of jobs lost and the cost of the new Medicare prescription drug bill all have combined to raise serious questions about Bush’s credibility and character. In a poll taken earlier this month by the Pew Research Center for the People and the Press, 21 percent of respondents called Bush a “liar” when asked to describe him—a trait that didn’t even register in polls taken in May of 2003.

On top of that, while an overwhelming majority of Americans still strongly believe that their country and the world are safer with Saddam removed from power, support for the war in Iraq is slipping. Last month, according to the Pew survey, 65 percent believed that invasion was the right course of action; now, that number is down to 56 percent. One month ago, 75 percent of those surveyed believed things were going very or fairly well in Iraq; now, only 63 percent say that.

This does not mean that Americans want to elect an anti-war or dovish president. What it does mean is that more and more voters are becoming open to the idea that Bush took the country to war for ideological, political, and maybe even personal reasons—not for what’s best for Americans’ security. This fall, the debate over Iraq, then, won’t be about whether or not invading was the right decision, but how that decision was made, how it was defended to the American people, and how the president handled the war’s aftermath. Bush will have to answer these questions convincingly if he has any hope of keeping the people’s trust and winning re-election.

On the other side, Kerry’s challenge will be to overcome the deep-seated belief that Democrats are soft on defense—unwilling and unable to use military force to keep America safe. Kerry’s initial answer to this charge has been to tout his record—not his nearly two decades on the Senate Foreign Relations Committee, but his combat experience in Vietnam.

Kerry’s service on a Navy boat in the rivers of Vietnam has been one of his most potent advantages during the nomination fight. After losing seats in the House and Senate in 2002 by ignoring the issue of national security, Democrats realize that they cannot do the same in 2004 and hope to prevail. That explains why Kerry’s Vietnam record resonates so well with voters looking for a candidate to beat Bush. As recent questions about Bush’s own spotty record in the National Guard have once again surfaced, Kerry appears to be the only Democrat who can bring the national security fight to Bush.

In the general election, in addition to his persuasive history of leadership during Vietnam, Kerry will have to add policies and rhetoric that convince voters that he can reprise this performance as commander-in-chief. Kerry must offer a forceful critique of Bush’s handling of the war on terrorism—including the invasion and occupation of Iraq—without appearing to be weaker than Bush about confronting America’s enemies. In addition, Kerry must explain his often confusing position on the war on Iraq (he first voted in favor of the resolution to authorize force, then against a later funding bill for Iraq, and has since been a forceful critic of Bush’s handling of the situation) to dispel the likely charge that he makes national security decisions for personal or political expediency.

Barring an “October surprise,” such as the capture of Osama Bin Laden, a major terrorist attack in the US, or a significant setback in Iraq, the contours of the security debate in this year’s presidential race have been set. Competing visions about America’s role in the world, its relations with its allies, and the future of the Middle East will all be judged through the prosaic but powerful lens of personal security—and which candidate seems most capable of delivering it.