With the primary and caucus season finally—and mercifully—upon us, an intriguing picture is coming into focus, one that will either continue a recent pattern or return us to an earlier period in American history.

On the Democratic side, the polls show a trio of senators—Hillary Clinton, Barack Obama and John Edwards—leading nationally or in early-voting states. Senators Joe Biden and Chris Dodd and Governor Bill Richardson trail not-so-close behind. 

On the Republican side, the polls paint a very different picture. Two governors and a mayor—Mike Huckabee, Mitt Romney and Rudy Giuliani—lead a pack of senators and congressmen.

So, if the eventual nominees emerge from these six leading candidates, which seems likely, given the front-loaded primary schedule, Americans will have a choice between a senator and a governor or a mayor who governed a city that’s as populous and as complex as most states.

Given recent history, that would appear to be good news for the GOP’s frontrunners. As Sam Karnick of the Heartland Institute observes, “Governors win,” especially when running against senators. It’s easy to understand why: Governors are proven executives, problem-solvers and consensus-builders. And as Karnick adds, “They don’t have a voting record on important and controversial national issues.”

But Huckabee, Romney and Giuliani should not take this to the bank. One factor that could reverse this trend—and open the Oval Office doors for one of 2008’s bumper crop of senators—is the growing importance of national security. 

Senators, vice presidents and generals—and sitting presidents, of course—win when national security is a factor. For example, in the early days of the republic, when survival and external threat were the defining issues, Americans chose generals, diplomats and senators to lead. In fact, five of the first eight presidents were generals or secretaries of state; the other three had served as VP, ambassador and/or senator.

Fast-forward to the 20th century, and a similar pattern emerges. As a global depression began to sow the seeds for a global war, Franklin Roosevelt defeated a governor in 1936. Then, in the middle of World War II, he defeated yet another governor in 1944. And as world war gave way to cold war, Harry Truman defeated a governor in 1948.

In fact, from 1948 to 1972, Americans elected precisely zero governors. Truman had been a senator, vice president and president. Dwight Eisenhower had been a general. John Kennedy had been a senator. Lyndon Johnson had been a senator, vice president and president. And Richard Nixon had been a senator and vice president.

One key reason for the upper chamber’s upper hand in the middle chapters of the 20th century was the Cold War, a national security issue that dominated national politics. Then, as now, senators grappled with issues of war and peace, protecting Americans from internal and external threat, building alliances, and crafting treaties.

But as America grew accustomed to the Cold War’s ebb and flow—and as Johnson and Nixon proved that D.C. didn’t have all the solutions—the country concluded that it was time to give an outsider a chance. And so, Americans turned the reins of power over to governors. Thus began a new era of gubernatorial dominance. Jimmy Carter, Ronald Reagan, Bill Clinton and George W. Bush would defeat an unelected president in Gerald Ford, two vice presidents in Walter Mondale and Al Gore, and a pair of senators in Bob Dole and John Kerry.

And to further underscore the broader point here, it pays to recall that at the end of the Cold War, with Europe’s eastern half teetering on the brink of revolution, Americans chose the trusted hand of George H.W. Bush, a VP, over Mike Dukakis, a governor. Once the Cold War was over, the country chose a governor over the elder Bush. 

But we no longer live in a decade about noting, or in an era when we have the luxury of debating the meaning of “is,” or in a world where every problem can be solved by mantras like “It’s the economy, stupid.” Just consider how the candidates have brandished their national-security credentials during the crisis in Pakistan, how much time they devote to defending where they once stood on Iraq and what they will do in Iran, or how much their debates are dominated by events outside the United States. 

In short, national security is, yet again, shaping and defining national politics. Some Americans believe the younger Bush, a governor, overreached in recent phases of the war on terror. Others believe his predecessor, also governor, underachieved in earlier phases of the war.

So perhaps the pendulum of presidential power will swing back to the upper chamber.