US Vice President Dick Cheney and Senator John Edwards take centre stage in a nationally televised debate that is likely to mainly focus on the Iraq war and US security issues.
With polls showing the race for the White House tightening after last week's clash between President George W. Bush and Democratic rival John Kerry, the stakes for the showdown between their running mates have been raised dramatically.
Mr Cheney, the administration's most outspoken supporter of the war and harshest critic of Mr Kerry, will make the White House's case for the invasion of Iraq and try to stem the momentum building for Mr Kerry since he put Mr Bush on the defensive in last week's debate.
Mr Edwards, a first-term senator from North Carolina who was chosen for the ticket by Mr Kerry after a strong primary run, will try to put the vice president on the defensive about the administration's policies in Iraq, its record on jobs and health care and its ties to special interests, Kerry aides said.
Mr Cheney is portrayed by critics as the dark architect of the Iraq war. His secretive energy task force, along with his ties to energy giant Halliburton and the oil industry, have made him a lightning rod for Democratic complaints.
Mr Edwards is expected to press Cheney about his role in formulating the administration's energy and Iraq policies, and on Mr Cheney's connections to Halliburton, which he headed from 1995 to 2000 and which now is a leading US military contractor in Iraq.