The US supports Israel because most American Christians support Israel, and not because of the small number of Jewish voters in the US. That’s another reason why Walt and Mearsheimer are wrong too.
If AIPAC and related groups call politicians anti-Israel, the tens of millions of non-Jewish voters who connect Israel’s security with American values and interests will believe them. (A recent poll found that 53% of voters were more likely to vote for a candidate who was ‘pro-Israel’.) AIPAC is powerful because it is the accredited watchdog on an issue the non-Jewish public cares about; if the dog barks, something is wrong.