While President Obama won the second debate marginally, the victory has meant little change in the polls; it seems the momentum Governor Romney’s first debate performance continues his surge in many public-opinion surveys. The third debate offers the president one last chance to score a decisive win that could swing the momentum back toward him. Bob Schieffer, the moderator of the final debate, announced the topics: "the changing Middle East and the new face of terrorism,” "our longest war—Afghanistan and Pakistan," "red lines—Israel and Iran," and "the rise of China and tomorrow’s world” and "America’s role in the world." Standing in the president’s way of changing the momentum is the topic of the Benghazi attack.

In a strange way, Governor Romney’s mishandling of the issue may have helped President Obama win the second debate, but it raised the stakes of the issue higher with voters. The president enters this debate with a slight polling edge on which of the two candidates is most effective at handling foreign policy—but Benghazi has cut into his edge. If Governor Romney can shift the focus of the debate about Benghazi on the president, the third debate could strengthen his momentum down the home stretch.

Will Governor Romney be able to force President Obama to answer the questions about what he and his administration knew and when they knew it? In particular, did the administration disregard requests for additional security in Benghazi? Or will the president be able to effectively argue that Governor Romney is politicizing a national-security issue for his own gain, as he did in the second debate? The answers to those questions could prove decisive—both Monday night, and on Election night.

So what do both men need to do? First, President Obama has to go on the offensive. He needs to be presidential, and not appear as just another candidate. His win in the second debate was generated from this posture in the Benghazi moment. He must keep the debate focused on foreign policy and his tactical successes—reminding voters he has killed more Al Qaeda in the past four years than the Bush administration did during its entire tenure, and that he killed Osama bin Laden. Watch for how Romney handles these claims—whether he is able to place them in a broader context. If not, Obama may use them to swamp discussion of Benghazi.

Governor Romney must make the debate about the economy, tying the foreign policy topics back to Obama’s vulnerable record on this front. He must argue that failures in Iran have a potential economic impact; our policy toward China is about our debt; and global trade and competitiveness affects our foreign-policy leadership.

Afghanistan, and the timetable for withdrawal, will once again be a flash point. The country is key to Obama’s assertion that he has wound down two wars, and he is expected to stick to his hard deadline for withdrawal. Romney needs to stress the unfinished business there, and the ongoing threat to American lives, if he is to score points here. He may also try to put Obama on the defensive by suggesting that a firm withdrawal date only aids and abets the enemy.

Look for Romney to try to put Obama on the defensive on Israel and Iran. If Governor Romney can frame the president’s handling of Iran’s quest for a nuclear weapon and the strained relationship with Israel as meaningful issues in an election dominated by jobs and the economy, then he may come away the victor. Romney forces Sunday morning sought to paint Iran’s offer of one-on-one talks with the U.S. as a temptation to turn our backs on our partners in pressuring President Ahmadinejad, while Obama’s camp touted its efforts to lead an international coalition pressuring Iran’s economy.

China offers Romney one of his better opportunities to link foreign policy and the economy.

Both men must show the mettle to be commander-in-chief—through not just what they say, but how they say it. Much has been made about how both of them projected a tone that was inappropriate for a president in the last debate. Governor Romney has to return to the confident-but-respectful aggressive style that helped audiences shape a new perception about him in their first showdown. And the president has to attempt to tease out from Romney the less commanding tone evident in the second debate. This is our last chance for us to judge who will make the best president. We will do so based on their competence in foreign policy, but we will also be judging them on how we think they will handle the most difficult of scenarios when, in Hillary Clinton’s famous construct, the phone rings at 3 a.m.

History offers some guidance. Questions about Ronald Reagan’s limited experience and the misperception that he would be a warmonger were answered effectively in his debate with President Carter mainly on how he presented himself rather than what he said. John Kerry won the Miami debate but failed to pass his “global test” and President Ford failed the foreign policy test with his proclamation that Eastern Europe wasn’t under Soviet domination. While the economy is clearly the most important issue in this election we will make important judgments about both of our candidates in how they might lead our country and help shape its place in the world during the next four years.

The stakes are high for this debate. Each candidate has a debate win so a victory in the final debate will set the narrative for the rest of the campaign of who has the momentum heading into Election Day. But more important, we may discover what happened in Benghazi, answers deserved by all Americans and by the families of those who were lost, and if the events of that fateful night will have a decisive impact on the presidential race, even if unintended.