The exchange between President Bush and Senator John Kerry could be crucial in a tightening contest.
This debate will center on domestic and economic policy. The candidates may spar as well over the war on terrorism and the war in Iraq, as they did in the other debates. But analyst Earl de Berge expects them to focus on subjects like public pensions and the Medicare health plan for seniors.
"The principal issues are health care, social security, Medicare, how tax policy will be an engine for economic development and the creation of more jobs, and environment," he said.
The Arizona pollster says 15 percent of American voters may be undecided, and each candidate hopes to sway them in his direction. Others pollsters put the undecided vote as less than five percent.
The candidates are addressing a national audience. By holding the debates in three sections of the country, they are also appealing to voters in those regions. The first debate was held in the southeastern state of Florida, and the second in the Midwestern state of Missouri.
Polls show that Arizona, the site of the third debate, is strongly in the Bush camp, but nearby Colorado, New Mexico and Nevada could go to either candidate. Pollsters say that nationally, the race is getting closer.
Democratic Arizona governor Janet Napolitano says the debate allow the candidates to reach out to the voters.
"And I think in a way, the topics of this debate may be the decisive topics for the whole thing," she said.
Pollster Earl de Berge says the debates so far have focused on important issues, and surveys show the voters are responding.
"I do think the debates have been exemplary in terms of bringing candidates forward to talk about issues and do eyeball contact with the American voter," he said. "And I think the proof of that is there is a huge amount of interest in the outcome, and people are really taking this seriously."
More than 60 million people watched the first debate. Nearly 50 million watched the second.