Xymphora makes that argument here: http://xymphora.blogspot.com/2006/12/no-comparison.html
Counterarguments? Are there any? One thing is certain: few Americans were ever attacked, abused and harassed by apologists for the former apartheid regime of South Africa. Representatives of the apartheid regime didn't conspicuously dominate the American political and cultural system, or drive the United States into a self-destructive and disastrous war, like that in Iraq. Which means: Israel has the potential to get itself into much worse trouble with Americans than South Africa ever did. Oh, the irony of it all.