There was a report, Sindlinger and Company Report (1960), that said that those who had watched the debate thought Kennedy had won and those who had listened thought Nixon had won. However the report fails to take any demographic information into account. For example those who were more likely to own televisions were more likely to be in the North East so were more likely to vote for Kennedy anyway. Those who only owned radio sets were more likely southern suggesting they were generally more republican anyway. Therefore the report was skewed.
General histories also fail to talk about the other two debates that happen. Studies of the whole debate series suggests that Kennedy won the first, Nixon the second and the Third was a draw. Whilst less american's watched the second two debates, the debate series was still drawn.
Further than this there were many other things that helped Kennedy to victory. Think about the differences in stance on the Vietnam War and the King's public backing of Kennedy's campaign.
Whilst the television would have undoubtedly swung a few voters it would not have had the large scale change in opinion needed for Kennedy's landslide victory. Kennedy had won the election before the television debates had been televised in all likelihood, they just aided to consolidate victory.
All of these are good sources for further reasearch: Busch, Andrew E., Reagans Victory: The Presidential Election of 1980 and the Rise of the Right (Kansas: University of Kansas Press, 2005) Davies, Philip John, Elections USA (Manchester: Manchester University Press, 1992) Druckman, James N., ‘The Power of Television Images: The First Kennedy-Nixon Debate Revisited’, The Journal of Politics, 65 (2003), 559-571 Roberts, Robert North and Scott J. Hammond, The Encyclopaedia of Presidential Campaigns, Slogans, Issues and Platforms (Westport: Greenwood Press, 2004) Rosenbaum, Martin, From Soapbox to Soundbite (London: Macmillan Press Ltd., 1997), Rosenstone, Steven J., ‘Why Reagan Won’, The Brookings Review (1985), 26-32 Schroderer, Alan, Presidential Debates: Fifty Years of High Risk Television, 2nd edn (New York: Columbia University Press, 2008) Hamilton, Neil A., Presidents: A Biographical Dictionary (New York: Check Mark Books, 2001) Jamieson, Kathleen Hall, Packaging the Presidency, 3rd edn (Oxford: Oxford University Press, 1996) Mervin, David, The President of the United States (Hertfordshire: Harvester Wheatsheaf, 1993) Pitney, John. J., The Art of Political (Norman: University of Oklahoma Press, 2001) West, Darrell M., ‘Television Advertising in Election Campaigns’, Political Science Quarterly, 109, (1994), 789-809 White, Mark, ‘Apparent Perfection: The Image of John F. Kennedy’, History, 98 (2013), 226–246. White, Theodore H., The Making of the President 1972 (Atheneum Publishers: A Bantam Book, 1973) Witcover, Jules, Marathon: The Pursuit of the Presidency 1972-1976 (New York: The Viking Press, 1977)