In 2004, the University of Michigan’s American National Election Studies (ANES) survey asked about 1,200 American adults to give their thermometer scores of various groups. People in this survey who called themselves “conservative” or “very conservative” did have a fairly low opinion of liberals — they gave them an average thermometer score of 39. The score that liberals give conservatives: 38. Looking only at people who said they are “extremely conservative” or “extremely liberal,” the right gave the left a score of 27; the left gives the right an icy 23. So much for the liberal tolerance edge.
The bottom line is that there is simply no comparison between the current hatred the extreme left has for Messrs. Bush and Cheney, and the hostility the extreme right had for Messrs. Clinton and Gore in the late 1990s. ~Arthur Brooks 
This seems like a lousy way to measure tolerance of differences. I may be unrepresentative (no, in fact, I am unrepresentative), but take this for whatever it’s worth. I grew up with Clinton hatred. If you were young and exposed to a lot of conservative media in the ’90s, it just came naturally to distrust and loathe these people. I certainly still strongly dislike both him and his wife, but I guarantee that my “thermometer” score for Bush and Cheney would drop below whatever it would be for the Clintons. It does seem to be true that more progressives are furious with Bush and Cheney than conservatives were furious with Clinton and Gore, but then that might have something to do with the respective policies of the two administrations.
There are plenty of things in the last administration to find fault with, trust me, and whatever I say about the Clinton administration is just in comparison with the current administration, but it is difficult, even as a conservative, to view Clinton and Gore more negatively when they objectively presided over a relatively more conservative government overall. Some of this was imposed on them by the voters in the 1994 election, but comparatively the last seven years have simply been worse in most important respects. If that’s how I see it, how much much more true would that be for progressives who regard Bush (incorrectly) as a crazed right-winger? Brooks is prepared for this, but gives a response that isn’t entirely persuasive:
Yes, Mr. Clinton may have been imperfect, but Mr. Bush — whom people on the far left routinely compare to Hitler — is evil. This of course destroys the liberal stereotype even more eloquently than the data. The very essence of intolerance is to dehumanize the people with whom you disagree by asserting that they are not just wrong, but wicked.
But how many people on the left actually compare him to Hitler? This is a favourite of some in the netroots and the crazed world of ANSWER and MoveOn, but how representative are they really? There were more than a few (not always inapt) Nazi references in conservative circles when Clinton was pushing his gun control legislation or after Waco. It may be that there is simply vastly more vilification of Bush, but if you were trying to make a case for relatively greater liberal irrationality and intolerance I can scarcely think of a worse way to show it.
Probably a much more relevant measure of tolerance of different ideas would be to look at how left and right respond to dissenters in their ranks. You could try to make an argument that progressives are more intolerant of war supporters than conservatives are of war opponents and cite the example of the netroots’ campaign against Joe Lieberman (the Purge of Joe Lieberman!), but that would end up being pretty unpersuasive, since the lone antiwar candidate on the right is widely loathed and shunned by a majority of conservatives and the actual antiwar candidates on the left are trailing behind a woman who voted for the authorisation resolution. Even granting that Clinton has now adopted an antiwar pose, she is most reluctant war opponent you’ll ever see.
Looking beyond foreign policy, you could find more ideological rigidity on the left, and these days you could argue that the right is overflowing with dissenting views on all kinds of things (or you could see this simply as evidence of chaos and confusion), but it seems to me that it is in policy debates and arguments over ideas (or the lack thereof) that you’re going to find the real proof of tolerance or intolerance. In the end, it will probably depend on the issue or policy. On abortion, pro-lifers on the left keep a lower profile than pro-choicers on the right, and on trade free traders try to stamp out any hint of opposition on the right. Tolerance of differences tends to decline as you approach core beliefs, or positions that are considered untouchable or inviolable. It is human nature to be more tolerant in those areas that are less important to you, and perfectly normal to be less compromising and accommodating over things you believe are fundamental. In this sense, evidence of greater “intolerance” of this sort is also possibly evidence for firmer conviction.