Blending into US society, including marriage to non-Jews and a tendency to look on Judaism more in religious terms than ethnic ones, is part of what's happening, the study found.
"For our parent's generation, the question that mattered was, how do we regard Israel? For Generation Y (born after 1976) the question is indeed, why should we regard Israel?"
said Roger Bennett, a vice president of The Andrea and Charles Bronfman philanthropies, which sponsored the study.
"Until people recognize that a healthy and animated dialogue about Israel is the first step to a meaningful connection, the 'Israel debate' that takes place in America is liable to become moot well before Israel celebrates its 100th birthday," he added.
US support backed by a vocal and politically powerful Jewish lobby has been a key feature of the Jewish state's success since its founding in 1948, an event that is widely
backed by US Jews and non-Jews.
But the study found that "feelings of attachment may well be changing as warmth gives way to indifference, and indifference gives way even to downright alienation."
The study found only 48% of US Jews under age 35 believe that Israel's destruction would be a personal tragedy for them, compared to 77% of those 65 and older.
In addition, only 54% of those under the age of 35 are "comfortable with the idea of a Jewish State" as opposed to 81% of those 65 and older.
It did find higher levels of support among US Jews, regardless of age, who had visited Israel.
There are perhaps 6 million Jews in the United States, only about a third of them affiliated with a congregation. Of those who do attend a synagogue, perhaps 40% are classified as liberal Reform, 32% middle-ground Conservative and 8% Orthodox, according to surveys.
The findings were based on a representative sample of 1,704 non-Orthodox Jews in 2006 and 2007 contacted in writing. Its error margin was plus or minus three percentage points. The authors said they excluded Orthodox Jews because they tend to be overwhelmingly supportive of Israel.
In general US Jews "have increasingly adopted the American idea of what it means to be Jewish—primarily a religious identity," Steven Cohen of Hebrew Union College,
co-author of the study, said in an interview.
"The decline in attachment is widespread. It doesn't depend on how you measure it," Cohen added, with the disengagement no different among political liberals or conservatives.
Cohen also said inter-marriage with other faiths had a strong impact with "younger Jews being much more likely to be married to non-Jews."
The trend is part of a long-term historic slide not likely to be reversed since "people do not seem to significantly grow in their attachment to Israel as they age," the study said.
'Assimilation is the biggest problem'
Steven Bayme, director of contemporary Jewish life for the American Jewish Committee, a major pro-Israel lobbying group, said "assimilation is the biggest problem" with declining support among US Jews, but he said it is not new.
"People growing up where there always has been an Israel" are more detached, he said.
But the study breaks new ground, he added, in finding that politics do not underpin declining support—that it is not as many assume a response to Israel's handling of the peace process or problems with religious pluralism in the country.
Bayme also said that even though Orthodox Jews were not included in the study, the segment represents "strong signs of Jewish renewal"—children involved in the Hebrew faith in numbers far disproportionate to those of non-Orthodox families and who in the future will help counter the effects of assimilation outside their ranks, he said.