is a political scientist and the president of the University of Pennsylvania

We are living in two worlds, politically speaking.

And we need to bring the two closer together, or risk forfeiting much of our potential as a democracy.

There is the world of polarized political rhetoric, which exposes us to a smashmouth culture in which extreme rhetoric dominates public debate to the point of hijacking it. Scare tactics, personal attacks, code words and distortions mobilize the base, in almost total disregard of the public good.

Reach across the aisle to explore the possibility of common ground or compromise - try to economize on partisan disagreements - and you are likely to be ignored or dismissed as indecisive. That's if you're lucky. More likely, you will endure malicious attacks on your character.

Most citizens live most of their lives in a much different world. They despair of a political discourse that crosses the line from partisan, passionate debate to extremist, character-assassinating rhetoric, the verbal equivalent of mud wrestling, but with far less demonstrable skill. They suspect that America's political system has grown so chronically dysfunctional that it is incapable of intelligently addressing, let alone solving, our most challenging, highest priority problems - from health care to global warming, from public education to Social Security, from terrorism to immigration and our eroding competitive advantage in the global economy.

We have reason to link the impoverished, divisive discourse that pollutes our politics with the diminished problem-solving capacity of our political system. Extremist rhetoric undoubtedly has eroded trust and mutual respect, which not only bodes ill for American democracy but also has deleterious consequences for public policy.

Let's consider two examples of this troubling symmetry: the debate over heath-care reform in 1994, and the debate over the current war in Iraq.

In 1994, 37 million Americans lacked any kind of coverage. In many ways, the Clinton administration doomed its cause at the outset by crafting a comprehensive plan in secret, thereby blocking public deliberation on the merits of their case while also arousing suspicions. Debate quickly disintegrated into a round-robin of recrimination, extremist obfuscation, name-calling, and fear-mongering. A flurry of attack ads designed to mislead or scare the public saturated the airwaves. One spot likened the Clinton plan to "involuntary euthanasia."

In a perfect storm of tit for tat, any chance for meaningful progress capsized; the Democrats lost both houses of Congress that fall. As of 2008, it is estimated that more than 47 million Americans lack heath-care coverage.

Then there is the war in Iraq. In Congress and in the Washington news corps, the initial case for going to war was rarely challenged or even questioned. There was no genuine deliberation and no real debate.

Critics and skeptics who did raise objections and concerns were charged with being "appeasers" or "soft on terrorism." Now, more than four years later - with more than 3,900 U.S. soldiers killed as of Jan. 12, according to iCasualties.org, and many more seriously injured, and tens of thousands of Iraqi casualties - control of Congress has switched parties. But the public debate is so thoroughly laced with polarizing rhetoric and mutual contempt that consensus or even a reasonable compromise appears almost impossible.

We face no shortage of difficult and potentially explosive issues ahead. In this age of terrorism, can we strengthen our strategic role by cultivating stronger relationships with our allies, and increase our economic competitiveness in the world?

Are we politically capable of a respectful compromise on immigration? Can we move forward as a society on stem-cell research? Can we address the fiscal problems of Medicare and Social Security or will we force today's college students to work mainly to support retirees? Will we prepare our nation to deal with the next disaster of Hurricane Katrina's magnitude?

We have a choice: We can allow the "war of the words" to polarize us even more as a nation and further erode government's capacity to address the urgent issues that face this nation. Or we can economize on our disagreements rather than exaggerate them, and infuse public discourse with a larger dose of robust but reasonable, nonpolarizing argument.

If we don't change course, the gulf between politicians and ordinary citizens will become an ocean, and the moderates - who represent majorities - will drown. No public good will be served, and politicians will lose any credibility as public servants.

Those are the stakes as we approach the 2008 national election, which will determine the course of our constitutional democracy in a complex world.