Matt Yglesias notes that left-of-center commentary has been notably more pessimistic on the state of the economy than right-of-center commentary. He offers three hypotheses (left-wing positions are psychologically correlated to pessimism, pessimism was correct, lefties wanted Bush to fail), all of which played a part. If you're interested in this subject, though, I'd recommend reading Larry Bartels' paper "It Feels Like We're Thinking: The Rationalizing Voter and Electoral Democracy."In it, Bartels and coauthor Christopher Achens examine simple factual questions for evidence of partisan interpretation bias. Their first example comes from the 1996 National Election Survey, which asked whether the budget deficit had increased or decreased during Clinton's first term. The correct answer what that it had decreased. By 90 percent. But they found that only one-third of the public recognized that the deficit had decreased at all. Republicans found the question especially tricky: More than half thought the deficit had increased.The next question asked whether the economy had improved or worsened during Clinton's term. During this period, GDP grew quickly, unemployment dropped, and wages rose. By any measure, the economy had improved. But only one-third of Republicans agreed with that statement. And Republicans, unsurprisingly, were twice as likely as Democrats to say that it had worsened. Bartels and Achen go further, however, and break their results out by the voters' political information level. What they found was startling: The more an individual voter knew, the more they self-deceived. "Among the least well-informed respondents, neither objective reality nor partisan bias seems to have provided much structure to perceptions of the budget deficit," they dryly note. "Uninformed Republicans and Democrats were slightly, and about equally, more likely to say that the deficit had increased than that it had decreased." But travel up the information scale, and the situation dims. Partisan bias exerts its pull. Objective reality does not. 
The thing to notice there isn't even how accurate the perceptions are. It's how different they are. The economy had improved. Democrats, who had no trouble aligning the economic improvement with their favored candidate, saw that easily. Republicans, who felt tension between the candidate they loathed and the economic news, tended to be much more tepid. Indeed, at the top of the information scale, Republican perceptions of the economy nosedive, presumably because they had enough information to construct a mental model of how the economy was actually doing badly (trade deficit, China, etc). And this is not a partisan point: If if George W. Bush had been in office during that period, the lines almost certainly would have reversed, but the distance between them would not. This is part of what makes political persuasion so difficult. Partisans don't simply disagree on the merits of candidates. They disagree on the shape of the underlying reality. And more information, because we tend to sort our information to strengthen our biases, aids our personal deceptions rather than correcting them.
The thing to notice there isn't even how accurate the perceptions are. It's how different they are. The economy had improved. Democrats, who had no trouble aligning the economic improvement with their favored candidate, saw that easily. Republicans, who felt tension between the candidate they loathed and the economic news, tended to be much more tepid. Indeed, at the top of the information scale, Republican perceptions of the economy nosedive, presumably because they had enough information to construct a mental model of how the economy was actually doing badly (trade deficit, China, etc). And this is not a partisan point: If if George W. Bush had been in office during that period, the lines almost certainly would have reversed, but the distance between them would not. This is part of what makes political persuasion so difficult. Partisans don't simply disagree on the merits of candidates. They disagree on the shape of the underlying reality. And more information, because we tend to sort our information to strengthen our biases, aids our personal deceptions rather than correcting them. 
