Even this early in the presidential campaign, it’s not difficult to envision the threshold issue for anyone seeking to move into the Oval Office on Jan. 20, 2009: Who can best keep America safe from terrorism?

The first round of nationally televised debates, which began with the Democrats last week and continues with the Republicans on Thursday, seems destined to be remembered mostly as a test of wills for measuring how the hopefuls would command the nation’s defense.

Sure, many other vital issues will also drive the electorate’s thinking, from health care to global warming. But there are none more fundamental to the role of a commander in chief than how to deal with enemies determined to kill Americans. The struggle against terrorism is even more pertinent as a voting issue than the fight against communism— the threshold test for White House hopefuls in the 11 elections during the Cold War. After all, the enemies in that conflict never attacked U.S. soil, as al Qaeda did on Sept. 11, 2001.

Given the underlying fear of terrorist threats, it was not surprising that a question about responding to more attacks produced the most telling moment of last week’s Democratic debate in South Carolina. Moderator Brian Williams of NBC News put forward a chilling hypothetical: “If, God forbid, a thousand times, while we were gathered here tonight, we learned that two American cities had been hit simultaneously by terrorists, and we further learned beyond the shadow of a doubt it had been the work of al Qaeda, how would you change the U.S. military stance overseas as a result?”

Most candidates punted, ignoring the military thrust of Williams’ haunting question. With a lone exception, their mushy answers stood in stark contrast to how most Republican presidential aspirants will probably answer a similar question, which they’re sure to get at the debate at the Reagan Presidential Library this week.

Amazingly, only Hillary Rodham Clinton showed the presence of mind to immediately say that, first and foremost, she would track down the perpetrators. “I think a president must move as swiftly as is prudent to retaliate,” the New York senator said. “If we are attacked and we can determine who was behind that attack, and if there were nations that . . . gave material aid to those who attacked us, I believe we should quickly respond.”

Other Democrats considered to be in the top tier of their party’s nomination race — Sen. Barack Obama of Illinois and former Sen. John Edwards of North Carolina — initially did not even allude to a military response, even though Williams had specifically asked them how they would “change the U.S. military stance” after an attack. Instead, they talked of analyzing homeland security failures and preventing an Iraq-style invasion based on faulty intelligence.

It speaks volumes about Clinton’s capacity to position herself against a potential Republican nominee that only she grasped right away how crucial it is for Democrats to rebut the inevitable claims that they are weak on defense. And yet she carefully measured her words to assuage those who recoil from George W. Bush’s Wild West rhetoric, promising to move “as swiftly as is prudent to retaliate.” Prudent retaliation could be just the doctrine that mainstream voters want after the debacle of the Iraq War.

Give Obama credit for at least belatedly realizing that only Clinton had directly answered in a way that foreclosed the familiar Republican portrayal of Democrats as wimps. He returned to the topic later on, in response to another question, saying that sometimes a military response is called for.

Sure, it is probably true that most Americans are weary and skeptical of Bush’s belligerent and provocative language about “the war on terror.” That was the defense many Democrats offered after the debate for the largely tepid response to Williams’ hypothetical. But it will not take much to remind Americans that the scenario of two U.S. cities under attack is not far-fetched.

We need not imagine how ferociously the Republican presidential candidates will respond to questions about defending the nation against more terrorist assaults. While most Democrats are playing to peaceniks among their core primary voters, GOP candidates are falling over each other to talk tough for their conservative base.

Before last week’s Democratic debate, former New York Mayor Rudolph Giuliani already had voiced how his party views the terrorist threat, taunting Democrats in a way that will surely be echoed until Election Day 2008. “The Democrats do not understand the full nature and scope of the terrorist war against us,” he said.

If Democrats keep up the ambiguous chatter about national security in the way that most did in their first debate, even a tough-talking Clinton might not be able to erase the weakling image that Giuliani was trying to establish.

Contributing Editor Craig Crawford is a news analyst for NBC, MSNBC and CNBC. He can be reached at ccrawford@cq.com.