The pattern has been clear since the 1930s. The only exception was Reagan vs Carter, in which it broke for Reagan.
Undecideds break for the incumbent, or the incumbent party.
http://myelectionanalysis.blogspot.com/2004/09/whither-undecideds.html
1996: Clinton -1, Dole +3 -- Net +4 Challenger
1992: Bush +0, Clinton -2 -- Net +2 Incumbent
1984: Reagan +2, Mondale +2 -- Net 0
1980: Carter +0, Reagan +6 -- Net +6 Challenger
1976: Ford +2, Carter +3 -- Net +1 Challenger
1972: Nixon +0, McGovern +2 -- Net +2 Challenger
1964: Johnson +5, AuH2O +3 -- Net +2 Incumbent
1956: Ike -1.5, Stevenson +1.5 -- Net +3 Challenger
1948: Truman +5.5, Dewey -4.4 -- Net +9.9 Incumbent
1944: Roosevelt +3.5, Dewey -2.5 -- Net +6 Incumbent
1940: Roosevelt +3, Wilkie -3 -- Net +6 Incumbent
1936: Roosevelt +5, Landon -7 -- Net +12 Incumbent
The single Carter-Reagan debate was October 28, 1980.
Former Carter pollster Pat Caddell claims he begged them not to do it so close to the election. Strategically, this was an extremely stupid move on the part of Carter's people, but doesn't stupidity sum up the entire Carter presidency?