There's a lot going on here.
On a shallow level, there's an element of family tradition involved -- Bubbie and Zaydie voted Democrat and they'd turn over in their graves if I voted Republican, etcetera -- but the issue can't be written off as a joke. The "Democrat, right or wrong" attitude bespeaks a fundamental discomfort among American Jews about their place in American society. For all their -- our -- apparently seamless integration, there's a thread of anxiety remaining: we have to be more American than the Americans, more of the people than the people themselves. And the Democratic Party, justly or otherwise, is perceived -- certainly within the educated, successful milieus where most American Jews are to be found -- as more of the people than the Republican.
To make matters worse, the Republican Party represents many of the characteristics reflexively applied to Jews. They're positive qualities, but they've been used against us for so long that many American Jews feel more comfortable aligned politically against them. They include a belief in the fundamental virtue of capitalism, a concern with fiscal responsibility, with keeping the government out of your pocket and out of your decision-making, with taking primary responsibility for your own success rather than expecting it to be bestowed from on high, with ensuring that your children's future is brighter than your own (and its corollary that you don't feed off a public trough and expect your children to pay for it). When applied to Jews, these positives have been twisted to impute greed, miserliness, selfishness (a disinclination to redistribute income or to spread the public wealth), callous disregard for the less fortunate, clannishness, and so on. Jews have been tainted with these slurs from time immemorial. It is perhaps little wonder that they flock to the party that purports to be all about inclusiveness and social rather than fiscal responsibility.
I'm generalizing here, but I think it's safe to say that as a group, American Jews have never felt absolutely certain of their acceptance as Americans. They don't just want to blend in; they want to be part of the family. Hence the high rate of intermarriage and flight from religious observance. Jewish Democrats lean heavily on their party affiliation as a means of expressing their attachment to the glorious melting pot: to the frontier of tolerance and freedom where at long last they can finally relax.Same deal here in Canada, with Jews leaning heavily on the Liberal party as a means of expressing their attachment to Pierre Trudeau's glorious multiculti tapestry. And yet in the last federal election, 62% of them voted Tory, the Republicans (more or less) of Canuckistan.
If Canucki Jews can get a clue, why can't American ones?