With the hours before Troy Davis' scheduled execution dwindling rapidly, the Georgia death row inmate's defenders made a last ditch attempt to delay his execution by demanding that prison officials let him take a polygraph test.

The movement to exonerate Davis has attracted an international following, rallying supporters from Amnesty International to Pope Benedict XVI to former President Jimmy Carter. But on Tuesday Georgia's pardons board for the third time rejected Davis' claim that his conviction for the 1989 shooting of off-duty police officer Mark MacPhail was based on faulty evidence.

Parole boards almost never grant clemency, so this is not a surprise, said CBS News Legal Analyst Andrew Cohen. Now if Wednesday's execution is going to be halted it's going to have to come from the federal courts, and the U.S. Supreme Court in particular, which last week halted a Texas execution.

Defense Team Unsuccessful in Invalidating Evidence

Davis' defense team has unsuccessfully argued that the evidence against him is tainted. Several of the witnesses who testified at his trial have since changed their testimony.

This is the fourth time that Davis has been within days of death: in 2007 his execution was halted the day before, in 2008 his death loomed 2 1/2 hours away when the U.S. Supreme Court intervened, and later in 2008 the federal court of appeals stopped the execution three days before he was to die.

We've been here before. We're just hoping it will go all the way through this time, Mark MacPhail Jr., who was an infant in 1989 when his father was murdered, told the Atlanta Journal-Constitution.

For his last meal, Davis has asked to be served the same fare as other inmates: a cheeseburger, potatoes, baked beans, coleslaw, cookies and a grape drink. Later in the night, while five reporters, Mark MacPhail Jr. and his uncle, William MacPhail, watch, Davis will be hooked up to IVs in both arms that will administer a lethal cocktail of drugs. Within five minutes, he will be dead.