Indeed the data don't support the concept of an inevitable second-term slump overall. The record instead is mixed: Dwight Eisenhower did better on average in his first term, but he was strong in both. Richard Nixon did far worse in his second term, given the Watergate scandal. But Clinton's approval rating averaged much better in his second term than in his first; likewise Reagan's, albeit more modestly so.
This ABC News/Washington Post poll was conducted by telephone March 2-5, 2006, among a random national sample of 1,000 adults. The results have a three-point error margin. Sampling, data collection and tabulation by TNS of Horsham, Pa.